Um, that's not very cool. You're asking people to invest their time and talent on the chance that their end product will be chosen above all others and an agreement will be reached on amount. All participants but one win..... maybe. No thank you. Not a very good way to treat your fellow photographers. That's spec work and it's not very popular in the design community and it's not very cool with post work. Either find someone who can deliver and enter into a contract with them or do as suggested above. Good luck.