Prompts are individually added. Registered members only? Prefer open. Make it a window of opportunity, and the spammers shouldn't find it.
Sign up to prompts, upload picture (or text) to them. Gallery of completed sketches (option for Muse Fusion??). Related gallery of pieces finished from the sketches.
Prompts sort by fewest signups, so stuff that hasn't been done is at the top.
Show author of prompts? Optionally.