I'm running into the same problem and can't figure out what's wrong for the life of me.
Whenever I view Who's Online (or any page that doesn't have a lot on it), the image stops at the bottom of the footer text.
I've read everything on the link previously provided. I've searched all over the internet and in-between. I've searched all over phpbb.com, and this is the only post that has hit the nail on the head concerning my problem. Any help would be greatly appreciated.
Problem -> http://gt.nitefalls.net/viewonline.php
No Problem -> http://gt.nitefalls.net/