http://www.hipforums.com/forums/showthread.php?t=44&highlight=signature+pics Check out the last post on the first page from the mod. It should help.
I had this problem too, you have to get a certain number of posts before you can post links and pictures and stuff.
Im pissed that doesn't make sense, ok someone with like 30 something posts had a picture in their sig, like, not a link, but the actual picture.... so it cant be number of posts cuz mine doesnt work. are there any other threads on this?