change the button background image on click as we see in its default behaviour

Question

hi , 

i have a button & two images image1 and image2

what i want to set an image1 in its background and when ever it clicks image2 will become the background of the button .

so to set image in backgroud i use 

 <Button Height="62.722" VerticalAlignment="Bottom">

            <Button.Background>
                <ImageBrush ImageSource="/WpfApplication1;component/pic/screen3_enter_button01.png"></ImageBrush>
            </Button.Background>

	</Button>

what i do to change the image on click event.