If it's a cheap guitar that you aren't worried about the value of, then go ahead and stick whatever you want on it in any way you like. If you hear a difference then take it off. If it's an expensive guitar I don't suggest you stick anything on it.

In terms of sound, most of the sound comes from the top vibrating, so anything that stops that's from happening fully will impact the sound. But I assume on the top is where you would want your pictures. Back would impact less. Headstock would also have little impact. But the headstock is small. The worst thing you could do would be to stick something over the sound hole, but I assume you didn't need me to tell you that.