Like the related posts Sarah.

"...the option of using plain text for the related content or a more visually striking display that includes featured images:"

If you don't have featured images does it grab the first image in the post?

Just checked out my own question...

"A post’s featured image will appear as the thumbnail. If you haven’t set a featured image for the post, the feature will pull the first image in the body of the post. Thumbnails are generated as square as possible."