Thumbnails on Directory Category pages

I've seen a few people mention this question, but haven't yet found an answer so I wonder if you can help.

The featured image is showing up fine on the single listing page, but what I want is for each listing on a listing-category page to have an image with it.

For example on this page: http://bit.ly/N8qV5h - I would like each entry to have its thumbnail showing. Is this possible?