Assuming you want the image to appear on the right hand side of each tab, you need to modify the template for the RibbonTab (if you only want it on one particular tab, you should put it on that tab only, by giving the style a name and tying the tab to that style). In the style I've put together here, look for the line
:<Image HorizontalAlignment="Right" Height="32" Source="Images/LargeIcon.png" Stretch="Fill" VerticalAlignment="Center" Width="32" Margin="0,0,10,0" Opacity="0.43"/>
and replace it with the imagery you need.
<ribbon:RibbonWindow.Resources>
<Style TargetType="{x:Type ribbon:RibbonTab}">
<Setter Property="KeyTipService.IsKeyTipScope" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ribbon:RibbonTab}">
<Grid x:Name="MainGrid" ClipToBounds="True">
<Grid.Visibility>
<TemplateBinding Property="IsSelected">
<TemplateBinding.Converter>
<BooleanToVisibilityConverter/>
</TemplateBinding.Converter>
</TemplateBinding>
</Grid.Visibility>
<Line x:Name="OuterLeftLine" Stroke="{Binding ContextualTabGroup.Background, RelativeSource={RelativeSource TemplatedParent}}" StrokeThickness="2" X1="0" X2="{Binding TabHeaderLeft, RelativeSource={RelativeSource TemplatedParent}}" Y1="0" Y2="0"/>
<Line x:Name="OuterRightLine" Stroke="{Binding ContextualTabGroup.Background, RelativeSource={RelativeSource TemplatedParent}}" StrokeThickness="2" X1="{Binding TabHeaderRight, RelativeSource={RelativeSource TemplatedParent}}" X2="{Binding ActualWidth, RelativeSource={RelativeSource TemplatedParent}}" Y1="0" Y2="0"/>
<ScrollViewer x:Name="ScrollViewer">
<ScrollViewer.Style>
<Style TargetType="{x:Type ScrollViewer}">
<Setter Property="Focusable" Value="False"/>
<Setter Property="VerticalScrollBarVisibility" Value="Disabled"/>
<Setter Property="HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="CanContentScroll" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid x:Name="Grid" Background="{TemplateBinding Background}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" CanHorizontallyScroll="False" Grid.ColumnSpan="3" CanVerticallyScroll="False" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Margin="{TemplateBinding Padding}"/>
<RepeatButton Grid.Column="0" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Command="ScrollBar.LineLeftCommand" Focusable="False" HorizontalContentAlignment="Left">
<RepeatButton.Style>
<Style TargetType="{x:Type RepeatButton}">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="Focusable" Value="False"/>
<Setter Property="Width" Value="14"/>
<Setter Property="Background" Value="{Binding (ribbon:RibbonControlService.Ribbon).Background, RelativeSource={RelativeSource Self}}"/>
<Setter Property="BorderBrush" Value="{Binding (ribbon:RibbonControlService.Ribbon).BorderBrush, RelativeSource={RelativeSource Self}}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border x:Name="OuterBorder" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1" Background="{TemplateBinding Background}" CornerRadius="3,0,0,3">
<Border x:Name="InnerBorder" Background="#80FFFFFF" CornerRadius="3,0,0,3">
<Path x:Name="GlyphPath" Data="M4,0L0,3.5 4,7" Fill="Gray" HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Border>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="HorizontalContentAlignment" Value="Right">
<Setter Property="Data" TargetName="GlyphPath" Value="M0,0L4,3.5 0,7"/>
<Setter Property="CornerRadius" TargetName="OuterBorder" Value="0,3,3,0"/>
<Setter Property="CornerRadius" TargetName="InnerBorder" Value="0,3,3,0"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="False"/>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="InnerBorder" Value="{Binding (ribbon:RibbonControlService.Ribbon).MouseOverBackground, RelativeSource={RelativeSource Self}}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="False"/>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="InnerBorder" Value="{Binding (ribbon:RibbonControlService.Ribbon).PressedBackground, RelativeSource={RelativeSource Self}}"/>
</MultiDataTrigger>
<DataTrigger Binding="{Binding HighContrast}" Value="True">
<Setter Property="CornerRadius" TargetName="OuterBorder" Value="0"/>
<Setter Property="BorderBrush" TargetName="OuterBorder" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
<Setter Property="Background" TargetName="OuterBorder" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
<Setter Property="CornerRadius" TargetName="InnerBorder" Value="0"/>
<Setter Property="Background" TargetName="InnerBorder" Value="Transparent"/>
<Setter Property="Fill" TargetName="GlyphPath" Value="{DynamicResource {x:Static SystemColors.ControlDarkDarkBrushKey}}"/>
</DataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="True"/>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="InnerBorder" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="True"/>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="BorderBrush" TargetName="OuterBorder" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/>
</MultiDataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</RepeatButton.Style>
<RepeatButton.Visibility>
<MultiBinding ConverterParameter="0" FallbackValue="Visibility.Collapsed" UpdateSourceTrigger="Default">
<MultiBinding.Converter>
<Microsoft_Windows_Controls_Ribbon_Primitives:RibbonScrollButtonVisibilityConverter/>
</MultiBinding.Converter>
<Binding Path="ComputedHorizontalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="HorizontalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentWidth" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportWidth" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
</RepeatButton>
<RepeatButton Grid.Column="2" CommandTarget="{Binding RelativeSource={RelativeSource TemplatedParent}}" Command="ScrollBar.LineRightCommand" Focusable="False" HorizontalContentAlignment="Right">
<RepeatButton.Style>
<Style TargetType="{x:Type RepeatButton}">
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="Focusable" Value="False"/>
<Setter Property="Width" Value="14"/>
<Setter Property="Background" Value="{Binding (ribbon:RibbonControlService.Ribbon).Background, RelativeSource={RelativeSource Self}}"/>
<Setter Property="BorderBrush" Value="{Binding (ribbon:RibbonControlService.Ribbon).BorderBrush, RelativeSource={RelativeSource Self}}"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type RepeatButton}">
<Border x:Name="OuterBorder" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1" Background="{TemplateBinding Background}" CornerRadius="3,0,0,3">
<Border x:Name="InnerBorder" Background="#80FFFFFF" CornerRadius="3,0,0,3">
<Path x:Name="GlyphPath" Data="M4,0L0,3.5 4,7" Fill="Gray" HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Border>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="HorizontalContentAlignment" Value="Right">
<Setter Property="Data" TargetName="GlyphPath" Value="M0,0L4,3.5 0,7"/>
<Setter Property="CornerRadius" TargetName="OuterBorder" Value="0,3,3,0"/>
<Setter Property="CornerRadius" TargetName="InnerBorder" Value="0,3,3,0"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="False"/>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="InnerBorder" Value="{Binding (ribbon:RibbonControlService.Ribbon).MouseOverBackground, RelativeSource={RelativeSource Self}}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="False"/>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="InnerBorder" Value="{Binding (ribbon:RibbonControlService.Ribbon).PressedBackground, RelativeSource={RelativeSource Self}}"/>
</MultiDataTrigger>
<DataTrigger Binding="{Binding HighContrast}" Value="True">
<Setter Property="CornerRadius" TargetName="OuterBorder" Value="0"/>
<Setter Property="BorderBrush" TargetName="OuterBorder" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/>
<Setter Property="Background" TargetName="OuterBorder" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
<Setter Property="CornerRadius" TargetName="InnerBorder" Value="0"/>
<Setter Property="Background" TargetName="InnerBorder" Value="Transparent"/>
<Setter Property="Fill" TargetName="GlyphPath" Value="{DynamicResource {x:Static SystemColors.ControlDarkDarkBrushKey}}"/>
</DataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="True"/>
<Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="Background" TargetName="InnerBorder" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
</MultiDataTrigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="True"/>
<Condition Binding="{Binding IsPressed, RelativeSource={RelativeSource Self}}" Value="True"/>
</MultiDataTrigger.Conditions>
<Setter Property="BorderBrush" TargetName="OuterBorder" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/>
</MultiDataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</RepeatButton.Style>
<RepeatButton.Visibility>
<MultiBinding ConverterParameter="100" FallbackValue="Visibility.Collapsed" UpdateSourceTrigger="Default">
<MultiBinding.Converter>
<Microsoft_Windows_Controls_Ribbon_Primitives:RibbonScrollButtonVisibilityConverter/>
</MultiBinding.Converter>
<Binding Path="ComputedHorizontalScrollBarVisibility" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="HorizontalOffset" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ExtentWidth" RelativeSource="{RelativeSource TemplatedParent}"/>
<Binding Path="ViewportWidth" RelativeSource="{RelativeSource TemplatedParent}"/>
</MultiBinding>
</RepeatButton.Visibility>
</RepeatButton>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ScrollViewer.Style>
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas x:Name="BackgroundCanvas" HorizontalAlignment="Left" Height="0" Margin="0,1,0,0" VerticalAlignment="Top" Width="0">
<Rectangle x:Name="OpaqueRect" Fill="{Binding (ribbon:RibbonControlService.Ribbon).Background, RelativeSource={RelativeSource Self}}" Height="{Binding ActualHeight, ElementName=ScrollViewer}" Width="{Binding ActualWidth, ElementName=ScrollViewer}"/>
<Rectangle x:Name="OverlayRect" Height="{Binding ActualHeight, ElementName=ScrollViewer}" Width="{Binding ActualWidth, ElementName=ScrollViewer}">
<Rectangle.Fill>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#EEFFFFFF" Offset="0"/>
<GradientStop Color="#BBFFFFFF" Offset="0.1"/>
<GradientStop Color="#05FFFFFF" Offset="0.5"/>
<GradientStop Color="#20FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
<Rectangle x:Name="InnerOverlayRect" Fill="#60FFFFFF" Height="{Binding ActualHeight, ElementName=ScrollViewer}" Width="{Binding ActualWidth, ElementName=ScrollViewer}"/>
</Canvas>
<Image HorizontalAlignment="Right" Height="32" Source="Images/LargeIcon.png" Stretch="Fill" VerticalAlignment="Center" Width="32" Margin="0,0,10,0" Opacity="0.43"/>
<ItemsPresenter x:Name="ItemsPresenter"/>
</Grid>
</ScrollViewer>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="CanContentScroll" SourceName="ScrollViewer" Value="False">
<Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=ScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=ScrollViewer}"/>
<Setter Property="Canvas.Top" TargetName="OverlayRect" Value="{Binding VerticalOffset, ElementName=ScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="OverlayRect" Value="{Binding HorizontalOffset, ElementName=ScrollViewer}"/>
<Setter Property="Canvas.Top" TargetName="InnerOverlayRect" Value="{Binding VerticalOffset, ElementName=ScrollViewer}"/>
<Setter Property="Canvas.Left" TargetName="InnerOverlayRect" Value="{Binding HorizontalOffset, ElementName=ScrollViewer}"/>
</Trigger>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding HighContrast}" Value="False"/>
<Condition Binding="{Binding ContextualTabGroup, RelativeSource={RelativeSource Self}}" Value="{x:Null}"/>
</MultiDataTrigger.Conditions>
<Setter Property="Stroke" TargetName="OuterLeftLine" Value="{Binding Ribbon.BorderBrush, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ribbon:RibbonTab}}}"/>
<Setter Property="Stroke" TargetName="OuterRightLine" Value="{Binding Ribbon.BorderBrush, RelativeSource={RelativeSource FindAncestor, AncestorLevel=1, AncestorType={x:Type ribbon:RibbonTab}}}"/>
</MultiDataTrigger>
<DataTrigger Binding="{Binding HighContrast}" Value="True">
<Setter Property="Visibility" TargetName="BackgroundCanvas" Value="Collapsed"/>
<Setter Property="Stroke" TargetName="OuterLeftLine" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/>
<Setter Property="Stroke" TargetName="OuterRightLine" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/>
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ribbon:RibbonWindow.Resources>