Here is an easy way to set it up:
create a folder, and inside, add your images. Name them 1.jpg, 2.jpg, and so on.
Then on the page you want to show the image:
Code:
<img src="images/rand/<?php echo rand(0,2); ?>.jpg" />
The "2" in rand(0,2) should be the total number of images you have.
Haven't tested it or anything, but should work.
