将图像添加到vb.net中的wpf按钮

i need to build a 15 picture puzzle and i don't know how to add an image to a button in a way that the image will fill all of the space of the button. or i should do it diffrently?? here is a link of what i need to build: sahek.co.il/content/full_screen.php?url=i need to build a 15 picture puzzle and i don't