We'd love to hear all suggestions about how to make this a fun, exciting new event.
I would imagine that part of the fun will be that many of the captions will be in parody--not of the artist, but of the scenario and/or characters depicted, and that they will be loving parodies, not sardonic or cruel. However, I concur such guidelines ought be spelled out aforehand, just in case. I think there could be a MST3K element to this event, and that could be really fun, so long as, again, parodying targeted the scene/characters, not the artist.
Would it be easier to maintain that delineation if the art were all fan art, or were you planning to open it to original character art as well?