Use case:

A user downloads many desktop backgrounds, opens Desktop Background Preferences, and drag-and-drops his backgrounds into the list. Then the user selects the desired background and closes Desktop Background Preferences. 

The user does not want all his backgrounds cluttering up his home folder, and since Desktop Background Preferences has functionality to add/remove files, then Desktop Background Preferences should "take ownership" of those files and allow the user to remove them from his home folder without removing them from the backgrounds list.

This could be implemented by copying all user-added backgrounds to /usr/share/background. Alternately, if disk space is an issue, a hard link could be created instead. Since this is a shared folder, file permissions become an issue. This is solved by making that user the owner of the file, and removing other users' write permission to that file (but still allow other users to read).

I am currently using GNOME 2.16, so if this is already implemented in 2.18 then my apologies! I would be able to write code for this, if necessary.

Thanks for taking the time to read this. :)