adding sponsor banners
 

Hi, I want to add sponsor banners to my site: www.acuarios.com.mx/foro.php


I want them to be placed exactly where this other site is placing them:

http://www.acuarios-marinos.com/forums/index.php

And I want the banners to change random and automatically everytime you enter a forum.

So how do you program that, and in which document should the code be written and between which lines?