I Just changed the thumbnail by duplicating the post, deleting the old one, and the new post has the first image as the thumbnail. If there is a rule that has been mentioned in the 671 posts, it would be nice to have it 'pinned' near the top of the comments somehow.
Background on my solution: I saved a post from a website, simplified the formatting, removed a bunch of images and the thumbnail was an image that I had deleted. I wanted a diff thumbnail, so I duplicated the post after removing all the unwanted content. The duplicate post used one of the images (the first one) remaining in the new post.