I used to be with OS 8-9.1 (OK, 9.2 for you folks out there with a new G4), you could simply drag a folder of pictures to the (open) Appearance control panel, with the Desktop tab selected, then click Set Picture. Then, whenever you restarted the Finder (tsk, tsk), restarted your computer, or logged out and logged back in (if you had Multiple Users set up), you'd get a different picture each time, randomly selected from that folder.
Alas, alack, that functionality disappeared with 10.0.x. I haven't seen any indication it's coming back, either.
