Simple Javascript is what you need. Type random image javascript into google. If you want to see this in action - look at
http://digitalet.net/forum I use this in conjunction with the welcome headers hack. Refresh a few times and you will see a variety of monkeys
I'd post the code but I'd have to go through my templates - and it's far to late for that. Maybe tomorrow if you don't find it in google you can pm me.