It helps if you use IE so you can see all the coding errors of a skin that FF typically ignores as FF simply wont display the broken image and make it look pretty for you
That is a issue with that style, you need to fix th following.
admincp --> vbulletin options --> Thread Display Options (showthread) --> Show Default Post Icon
Set the full path to this icon, ie /home/username/public_html/forum/images/icons/icon1.gif or try the domain yourdomain.com/forum/images/icon/icon1.gif
That should resolve your issue