In my recent re-design on Movalog, I had three banners and unfortunately I liked them all and couldn't figure out which one I liked best. As a result, I decided to use them all.
First I moved all the images to a subfolder called banner and renamed the images to 1.jpg, 2.jpg and 3.jpg. Finally I used PHP's mt_rand function to randomly print a number from 1 to 3 (mt_rand works better than the plain rand function function), like so
<?php echo mt_rand(1,3); ?>
Hence my banner image code looks like this
<?php echo mt_rand(1,3); ?> <img src="http://www.movalog.com/images/banner/<?php echo mt_rand(1,3); ?>.jpg" alt="Movalog - All Things Movable Type" />