Because you applied this hack to your VB3 forum. That code did not come with our download. We provide a link to it for people who want to use it.
We offer support for the unaltered Photopost distribution.
Here is a direct link to that hack
http://www.photopostdev.com/showthread.php?t=153
You see it is a hack to place the photopost cats and thumbs on your forum page. You can ask questions on the hack site if your having an issue with the hack. Basically your going to need to alter that hack links to the proper url from the spider url