I have the opposite problem: how do I prevent users from changing the greeter wallpaper?

We have a bunch of Ubuntu 12 boxes set up as web kiosks in our school library, and they are pretty thoroughly locked down. Any changes a user makes to the desktop environment will be wiped at logout - except, if a user changes their wallpaper, the change persists as the greeter wallpaper!

What file or directory do I have to lock or delete to prevent this?