Thank you for the reply, however, you completely missed the point.
It is not that the first image is the thumbnail, it is the first image is used to create the thumbnail.
The first image (or the image placed in H1) is the one used for the thumbnail, and is run through a resizing script on steemit.com
steemimg.com saves, automatically, three sizes of your photo. One is a thumbnail.
But, the only way, currently to use the thumbnail sized image, is to place the thumbnail sized image as the first thing in your post.
Which will usually jack up the looks of your post.