I'm curious how the database selects the photo for the main area pages? Is it the photo with the most "great" votes? Is it the first photo posted to the area? Is it determined by the person who submitted the area?
If there are no pics posted to the area, it seems that a picture from a random (or a "MP Classic" random) route gets placed there. I've been reorg'ing the Trapps, and I've got a placeholder page ("New Trapps"; the old Trapps content will be copied there eventually) for it, where I haven't put any pictures; yet a random one has shown up.
Otherwise, it's as Mono said - authors and admins can set a photo as 'first' which puts it up top, but by default the first-posted picture goes up there.