I had heard that there was a tag that was used in the past to select the thumbnail image, but some update a couple months or so ago removed that. My understanding is that the first image in a post with always be used as the thumbnail image now.

One small workaround for this though, is to put the image you want in thumbnail at the top of the post (it can even be smaller dimensions, followed by a <'hr> or a page divider image. Then put your main heading to "start the post." Most people don't see this as strange when they open the post up.

You can see an example where I had my friend do this: Here. I wanted to make sure the final colored pic was used for the thumbnail, but still wanted to have them all in sequence in the post.