Built this. Any item on the board whose post has an attached image now shows a small thumbnail on the left, as tall as the item, and clicking it opens the thread just like the title. Recent gets the same thumbnails, since the Recent thread asked for them too. There, each row shows its own post's image, so a reply with a picture shows that picture.
It's in review as https://github.com/gulkily/v3/pull/28 and isn't live until it's merged and deployed.