how to rotate banners after first post using php

[Kalina]

Hello,

I have a banner displaying under the first post in my threads. I want to make it into a banner rotation using php. I am using this script to rotate the banners, http://abledesign.com/programs/banner.php

I have that working but I don't know how to get it working in vbulletin so they rotate under the first post. I am running vbulletin 3.5.4.

I know I have to make a plugin and call it with a variable but I can't get it to work. The template I have to include it in is posbit_legacy.

Thanks for any help,
Colleen