Slide In - Thumbnails not showing

I'm using the slide in to display related topics for vehicle listings on my website at the bottom of the page. This is an automotive theme using a custom taxonomy "Listings" and I'm relating the vehicles by type (compact, pick-up truck, etc.)

I was thinking that the slide in plugin might use featured images (not supported in the Listings), and that's why they aren't showing. Is there a way I can get it to simply take the first image included with the post?