I see where that problem would rise up. To make it easier, perphaps of having each challenge an icon, FO banner, and header one, pick just one theme for it: like one challenge can be just for a FO banner, or just a header, etc. For making the banners, it would make it only 4 per challenge (first place, second place, third place, and mods choice).

Anyway, these are just some thoughts. Good luck :)