This is probably one of my biggest pet peeve when it comes to art sites. Some of the stuff in the Popular section of the site is not PG-13 (focusing on butts, lude poses). To me, that's indicating that the users of the site don't know the rules.
The MPAA's criteria for PG-13 should posted on the rules page and a link to it the rules page should be obvious on the upload page.
Customer support service by UserEcho