As a customer, how much creative control would you want to have over the finished piece?
I would expect the ability to send a headshot, desired role (ie, fighter, wizard), possibly gear subject to author approval. Then for the art to appear in the class writeup. No larger than a third of a page, based on the PHB.
It just seems like campaigns for game books have the unique opportunity for an offer like that. Or the chance to submit a name to be used for an NPC or spell/magic item.