Ghan is my system admin for my site and he created it for me, It's a simple php script that creates an image that goes in signatures, and then it pulls the banner images from my site which are located in a folder on our server.
When a game goes inactive, we just move it out and it leaves the cyle, or when a new game is added we add it to the pool of images. It used to be much smaller and less annoying because banners were different a while back but the entire template was changed, so the cycle might get reworked.
It is completely random from the pool each time the page is loaded, members from our site typically wear it in their signatures on other sites to help promote the site and the games on it, the images have a chance of looking appealing to someone and they might click on it and check it out.