banner exchange, how to?

Hi =) I'd like to setup a banner exchange with some friends. Some of them don't have PHP access, so I'm trying to come up with something that will work for everyone. I checked one site that's running a banner exchange and saw them using a cgi script like this: <a href="http://www.site.com/exchange.cgi?ad=NonSSI&page=XX"> <img src="http://www.site.com/exchange.cgi?id=siteB&page=XX"> </a> Looking at that, it suggests the cgi script is some how sending image data to be used for displaying the banner ad. Then, on top of that, it's some how routing the visitor to the correct location when they click on the ad. I don't see how they are doing it and figured I'd ask here to see if anyone has any ideas. If you're not sure of how they did it, any ideas on how to accomplish what I'm after? I came across another site that had the following code and some how they have it working as well. It would be cool to figure out how they did it. <IFRAME SRC="http://www.lemnet.com/banners/show/?user=qatlantis" HEIGHT="77" WIDTH="470" SCROLLING="no" MARGINHEIGHT="0" MARGINWIDTH="0" FRAMEBORDER="0"></IFRAME> <ILAYER SRC="http://www.lemnet.com/banners/show/?user=qatlantis" HEIGHT="77" WIDTH="470"></ILAYER> <br> <br>