Change VisualBrush in code behind?

Hi,
Thanks for your help.
 
I just want create a button Start/Stop with 2 differents images in background, one to Start and one to Stop.
 
Actually, I have a ressource dictionnary with differents images. These images are SVG file conversion.
I have a ressource dictionnary for styles. For my buttons:

<pre lang="xml">
!-- Controle Button -->
    <style x:key="CtrlButton" targettype="{x:Type Button}" xmlns:x="#unknown">
 
     
     
        <setter property="Template">
            <setter.value>
 
                <controltemplate targettype="{x:Type Button}">
                    <border x:name="ButtonBorder">
                              CornerRadius="10,10,10,10" 
                              BorderThickness="0,0,0,0" 
                              BorderBrush="#FF8F8F8F"
                              RenderTransformOrigin="0.5,0.5"
                              Background="{TemplateBinding Background}"
                         
                    >
                      
                        
                    <contentpresenter x:name="ButtonContentPresenter">
                             VerticalAlignment="Bottom"  
                             HorizontalAlignment="Center"
                            
                    />
                        
                    </contentpresenter></border>
                    <controltemplate.triggers>
                        <trigger property="IsPressed" value="True">
                            <setter property="RenderTransform" targetname="ButtonBorder">
                                <setter.value>
                                    <transformgroup>
                                        <scaletransform scalex="0.9" scaley="0.9" />
                                    </transformgroup>
                                </setter.value>
                            </setter>
                        </trigger>
                    </controltemplate.triggers>
                </controltemplate>
                
            </setter.value>
        </setter>
    </style>

And in my Xaml code :

<button style="{StaticResource CtrlButton}" name="btnStart" height="64" width="64" margin="50,20,10,0" click="btnStart_Click">
                    <button.background>
                        <visualbrush visual="{StaticResource Play}" stretch="Uniform">
                          
                        </visualbrush>
                    </button.background>
                </button>

 
With your code example, the image change only the mouse is over the button?
 
For my problem, i thought it was necessary to change the background image only by code. I would look in the xaml example code.