I have a button with an image defined in XAML like this:

```
<Button x:Name="buttonTest">
    <Button.Template>
        <ControlTemplate>
                <Border HorizontalAlignment="Center" VerticalAlignment="Center" >
                    <Image x:Name="imageTest" Width="57" Height="81" Source="/Images/sample.png" />
                </Border>
        </ControlTemplate>
    </Button.Template>
</Button>

```

How can I change the source of the image when the button is clicked?