What works will depend significantly on your user base. I've been in forums where voting just resulted in chaos but moderators worked well. Here voting works well and moderators would probably be an issue. So look at your user base and from that decide what will work best. I would definitly weight moderator and author votes much higher than user votes, maybe 10 unapposed votes from users would mark it unclean, where a 10/4 vote would make it undecided? There is probably some threshold in there that you will only find through trial and error, and it will all depend on your user base.

I don't know if there is an existing solution, but you might even see about generalizing it more and making it a tag system, where clean/unclean are just two tags, and others could be like/dislike, favorite, colors/subjects/medium/etc. Could make for a very interesting system if everyone can tag photos and different user roles have different weights in tagging.