Duh. Because the images would go walking around and vanishing and maybe even talking to you instead of saying their lines.
though, actually, the mobile pictures of the wizarding world might be fun but they aren't very useful for the basic purposes of pictures. Suppose you actually wanted a photograph of your family to show people. It would be awkward if one child's image was shy and ran off. And for historical purposes, you want an illustration that doesn't stop depicting what you want.
Sculpture can be stationary. why not flat images? How much magic does it take to do what Muggles can do with mere chemistry?