Maybe if the entrants pay to get their art considered, and they get rejected, there's something else to the deal, like they get a copy of the book or whatever. Personally I would try to jam every paying customer in there, just to be fair. Full pages for the good art, thumbnail pages for the bad. But if they have to limit the art to what's saleable, they could charge an entry fee that would cover the cost of printing all the entrants' copies, as well as the cost of printing some further number to cover some fraction of the purchases they'd expect from non-entrants.