A user above mention Microsoft's PhotoDNA. He could run the flagged images through that first and if it returns nothing maybe look at it.

Or don't look at it until a certain number of users flag it but still run it through the PhotoDNA. Now I am curious how imgur handles this problem.

Or use all of those CP databases, and just report images flagged by some subset of them.

I am curious how this gets handled. Having been goatse'd a few times with CP, I cannot imagine reviewing that crap on a regular basis.