With the exception of TMI, the tradition with box art has been to have it handled by a member of the game's art team. I would hardly turn down another Steve Purcell Monkey Island cover, but it would be highly appropriate for Rex Crowle to get the assignment as well.
It's worth noting that Purcell turned down the opportunity to do the cover of CMI because he was impressed by what the art team had come up with and felt they would better represent the style of the game.