I think this is a problem implicit to anonymous imageboards. By definition, you (as administrator or moderator) can't stop someone from posting CP, you can only be reactive about deleting it after you find it. The policy for moderation can be as rigid as possible but regardless it still requires the offensive content to be posted and discovered first.

One possible way to solve this would be to force images through some kind of premoderation process before being posted, but that would probably destroy the conversational flow of the threads, especially if the mods happen not to be awake and sorting through the queue at the moment.