Xstarfish is another tool you can use to set your screen's background image. However, unlike other wallpaper tools that simply paste pictures into the background, Xstarfish creates a new random image everytime it is invoked; you can also set it to periodically generate a images. I won't go into how to use it here; the man page is clear enough.
Urukrama's Weblog has a couple nice screenshots to give you an idea what it does.
A word of warning: if you go for the '-s full' option, it will eat a lot of cycles before it shows anything.
via Urukrama's Weblog