thecoalman - 8:29 am on May 16, 2010 (gmt 0)
You can use php, style.php :
$rand_num = mt_rand(1, 4);
background-image: url('randimage<?php echo $rand_num ?>.gif');
The first line is the most important because it will send the right header to the browser. The variable $rand_num is assigned a number between 1 to 4.
You'll have four images:
randimage2.gif ... etc