Change the background image of a Button when pressed using VisualStateManager

I have this button :

<Button x:Name="PrevAdIcon" Tag="-1" Visibility="Collapsed" Width="80" Height="80" Click="PrevAd">
    <Button.Background>
        <ImageBrush AlignmentY="Top" Stretch="None" ImageSource="/Images/prev.png"></ImageBrush>
    </Button.Background>
</Button>

How can I change the background to /Images/prev-selected.png when a user pressed the button ? It'll give him a feedback, since it's a WP7 app

what I have so far (not working) :

<vsm:VisualState x:Name="Pressed">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames BeginTime="00:00:00"  Duration="00:00:00.0010000" Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0">
                                                <DiscreteObjectKeyFrame.Value>
                                                    <ImageBrush ImageS开发者_StackOverflow中文版ource="/Images/prev-selected.png" Stretch="Fill"/>
                                                </DiscreteObjectKeyFrame.Value>
                                            </DiscreteObjectKeyFrame>
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </vsm:VisualState>

As far as I know, you can't change the value of the Source property on the Image element by using the VisualStateManager. However, you can just add two Image elements to the ControlTemplate: one for the Normal state and one for the Pressed state, and toggle the Visibility in the Pressed state.

<VisualState x:Name="Pressed">
    <Storyboard>
        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="Img">
            <DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed" />
        </ObjectAnimationUsingKeyFrames>
        <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)" Storyboard.TargetName="PressedImg">
            <DiscreteObjectKeyFrame KeyTime="0" Value="Visible" />
        </ObjectAnimationUsingKeyFrames>
    </Storyboard>
</VisualState>

See Peter Torr's post on "Why can't I change the Background of my Button on a Click event?" for an example and explanation of how to do this.

To add more information to Derek's answer, you should look at Gambit's answer here for full XAML that works