Desktop Drapes is a nifty utility used to randomize your desktop wallpaper. You add a couple pictures to the list, tell it how long to set them for, and you'll get some instant freshness to your desktop.
Drapes makes it even easier by automatically adding wallpapers you add to the directory. All you get to do is put the picture in the right place, and Drapes will figure it out for you. Open up a Terminal (or do it the Synaptic way) and type the following: