wont let me push set wallpaper

The only time I have seen a system not allow the changing of a wallpaper is if the computer is company owned or someone else has the admin account and they have blocked the ability to change the wallpaper. If that is the case here (either of them) you would need to talk to the owner (individual or company) before trying to change anything on the computer.

If that is not the case here, then please clarify exactly what you are trying to do and exactly what is happening when you try to do it.