So, you've downloaded from, say, GNOME Art, some nice background images for your GNOME 2.16 desktop, but they are only available to your account: if you use another login, they are nowhere to be found, because they got stored in the home directory of the first account. So you go back to the other login, and look for a way to transfer them to a central, system-wide, location, using super user privileges, but there is no tool to be found which will help you accomplish that.