Further, tho I didn't shoot much this year apart from some bees, I did spend a good bit of time helping dirtdorphins select her entries for the photo contest. And one criterion for that (given the nature of the site and the contest itself) was how the shot looked as a thumbnail. The question being, does this thumbnail make someone want to click on it so that they might eventually vote for it?
With that in mind, here's one that we rejected because it makes a disastrous, lopsided thumbnail that doesn't come close to describing what the picture is or what it's about:
This, however, might have drawn some clicks (were it not bracketed by the frame, especially, like on the site William linked):
I'm not saying that picture was a winner. Only pointing out that it was summarily rejected given the thumbnail that the site created for it. For the contest and perhaps even the database (in my mind, at least), those that are not cropped square with the subject in the direct center are essentially out of play.
YMMV, of course.