Try creating an ImageBrush in the Window.Resources, and set Window.Background to the resource.
<Window x:Class="WpfBackgroundImg.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{DynamicResource backgroundImage}"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<ImageSource x:Key="imgSource">image_resource.jpg</ImageSource>
<ImageBrush x:Key="backgroundImage" ImageSource="{DynamicResource imgSource}" Opacity="0.3"></ImageBrush>
</Window.Resources>
<Grid>
</Grid>
</Window>