Ask HN: How do I keep child porn out of my site?
Microsoft made an automated system (PhotoDNA) for detecting known child pornography images available to the public a few years ago and it's probably a good starting point:
<a href="http://www.microsoft.com/en-us/PhotoDNA/" rel="nofollow">http://www.microsoft.com/en-us/PhotoDNA/</a><p>Hopefully this can help you.<p>(Disclosure: I work at Microsoft but not on PhotoDNA.)