I have a problem of lots of people changing my desktop background. Is there a way that I can make people have to input an administrators password to change the desktop background. I am running ubuntu 12.04 lts with unity 2d.
I don't know what your are doing and why they change your desktop background but you could let the other users only use the 'guest session' (a default feature -> the login screen shows a guest session login button) or create a separate user account for them (System Settings -> User Accounts).
Another solution but more a dirty hack:
You could set the background using a cronjob every ~5 min: