yes, the first pic uploaded is the default (unless you change that in several ways).
so then, the default pic is always one of the 5 (a good thing),
and the other 4 are the last pics uploaded (be they from the poster or visits)......
kinda reminds me of the bug that time where someone would upload a pic to their log, then that pic became the default for the waymark--i know i had to edit quite a few to remove the 'gps pic' from the visit as the default pic for the mark....