I decided to try using the_post_format_image to grab the first image associated with a post and display it with the excerpt. However, if the post is a standard format it does not grab the first image - the entire post is displayed!
I'm not sure if I'm using the function as it was intended, but the description in media.php reads:
"Return the first image in the current (@global) post's content"
so I would expect only the first image found in the post to be rendered.