[setishock]

Copy this file > http://www.alistapart.com/d/randomizer/rotate.txt
and save it to notepad as > rotate.php
Next make a folder and put the images you want to use and the rotate.php file in it.
Upload that to your ftp and make note of the path to get to them.
Go in to the acp and navigate to the theme you want to setup and find doc_background in the stylesvars. Refer to the attached image. When you set the path it should be in this format > url(yourforum.com/path/to/images/rotate.php)
In the attachment I set the offsets to push the image down from behind the header and navbar. You may not have to do that but it helps to know how. Here's my setup using the php file > http://www.animeappeal.com/?styleid=111 You'll also note the Greetings! sidebar block makes use of the php file also. Click on the header or jump to another page. The images changes each time.