From Neal Stephenson's Cryptonomicon:
The background of his [X] screen happens to be a uniform indigo at the moment, but it could be an image. Theoretically you could use a movie ... You can, in fact, take any piece of software and make it into your screen background ...
How would you set a program as the desktop "wallpaper"?
You just run a program on the root window. Most screensavers allow you do do that, e.g.:
/usr/lib64/misc/xscreensaver/sundancer2 --root
However, most modern Linux environments run a "desktop" program that covers the whole screen and covers the root window. So you won't see it unless you kill the desktop program.
