In our site's "F.A.Q." link we explain that the ONLY way to change your Default picture (not the Thumbnail but the full-sized one you see when you click on that members profile) is to SWAP out the first photo you have in your group of photos.  I get emails from members asking if there is an easier way to make a photo as default other than SWAPPING it out with the first one listed.

Maybe if I can suggest a POSSIBILITY just add another link by [make thumbnail] when you are viewing the photo in LARGE format.