They don't pick it, it is an algorithm that has to do with views and ratings. Just cause you have the top rated picture doesn't mean you get on the front page (I was in a top rated picture for 3 days in a row and it never made the front page). Old pics are usually run if they're significant to current events (i.e. a skiers death). This system is also newish (since they revamped the pictures section a few months ago), before they were picked out by a staff member (and didn't change as often).