Thanks,
For future reference 4 anyone who wants to use custom thumbs, not rely on grabber:
Ok, figured that this is the code you have to use:
<th align="center" valign="middle" scope="col"><div align="center"><a href="tp/out.php?link=toplist&to=%%DOMAIN1%%"><img src="%%BANNER_SRC1%%" /></a></div></th>
In admin, edit each trade individually, and set the URL and thumb name for the thumb in the "Banner URL" box.