Time to make your life a little bit less predictable with Random Desktop. It will automatically choose a random desktop pattern and then choose a random picture from the folder that you specify (hard coded into the script). It is somewhat slow since it has to start up Desktop Pictures to run, and it quits Desktop Pictures even if it is already running, but it isn't so bad if it runs at start-up while your busy doing other things, anyway.
Random Desktop will only work on systems with Desktop Pictures installed. Appearance does not provide an easy way to choose random patterns, and it is impossible to set the desktop picture. Hopefully Apple will fix this in the future, but for now this is a Mac OS 8 only tool.
UPDATE: In Mac OS 8.5 and higher (I've only tested it in 9, though), you can drop a folder onto the mini Desktop image in Appearance filled with pictures. Then, one will be chosen, at random, from that folder at startup (of Appearance). Still no random patters, AFAIK, and you can't do any fancy tricks with the way the pictures are placed, but this is an improvement.
