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"The system looks for a number of complicated parameters and grades images at three levels, based upon their correlation with criteria that have been automatic into the system."

Unless there's something new and fantastic here that wasn't mentioned, this technology has been around for a good decade or so. They usually check for percentage of skin tones in the image, limb positioning, etc...

Just a couple things to look at from a brief search:

http://www.phpclasses.org/browse/package/3269.html
http://cat.inist.fr/?aModele=afficheN&cpsidt=18743518
http://serv1.ist.psu.edu:8080/viewdoc/download;jsessionid=92D8115F9C98B5DC6C8459E1A038AE46?doi=10.1.1.96.9872&rep=rep1&type=pdf
https://publications.lib.chalmers.se/cpl/record/index.xsql?pubid=76159

There's a whole field on this. If you do a bit of digging, you'll be able to find quite a bit more.

That first link is a package for PHP for detecting nudity. I'd start there.

Hrmm that might be a good feature to add. Screen out all of the really good stuff to send to the admins 

Some really interesting (and troubling) things can be done with image recognition software these days...