I'd like to give my two cents on the issue, as I used to be a forum admin myself, for a year. I used to have a rather small forum, with around an average of 10-20 active users per day. I used to do most of the moderating myself, with two moderators, one who didn't exercise his authority at all (he happened to be an expert on the subject matter, hence I appointed him). Going through this thread, I read that there are three moderators on wp, and currently around 100 members online! I think you guys are doing an amazing job, hats off to you.
As for spam attacks such as tubgirl images and such, I would suggest that you guys divide your time equally to moderate, according to timezones. For example, three moderators equals to eight hours each. Check every half an hour or so. I think that should be good enough to prevent such occurances in the future.
Btw, PPR is nothing compared to most forums I've been to. WP is a rather peaceful place, IMHO. Lots of Islamophobics here, however.