I really think it would be a good idea to define timing in the rules in order to make the game more fluid and effective... The only aspect related to timing in the current rules is that each challenge will run for a week at a time, but it doesn't mention what's the limit time to submit a cover, the limit time for the current winner to choose a new winner, and the limit time for the current winner to choose a new challenge.
All those periods should have a limit of days (maybe 3 sounds fair and enough?)... if the person disappears, someone who didn't play in the current round should have the chance of choosing a new winner our a new theme in order to make this thread active again. Otherwise is turns kinda frustrating for the ones who want to keep playing and have time for it.