Hmm. I've never used Slothtrader before and I checked the docs quickly for you and I don't see any mention of that feature.
http://www.slothtrader.com/help/siteconfig.html
I know tradepulse can do it for example like so:
Code:
<a href="/tp/out.php?to=%%DOMAIN1%%&link=top"><img src="%%THUMB1-1-1%%"></a>
TP grabs the thumb and then assigns it to be used in this way (%%THUMB1-1-1%%). Maybe slothtrader has no such mechanism built in? I think Rowan from ADX wrote Slothtrader so maybe ask him.
I'd imagine that you could do this on your own by writing code (if the script is open do it within, if not you'd have to do it in this way as external code) to parse the text toplist generated by slothtrader, then to grab thumbs, then to rewrite the new toplist with the thumbs, perhaps called by cron but that would be messy and a lot of work. It would probably be easier just to use Trade Pulse.
Good luck.