[daenz]

If there isn't already, maybe there should be some kind of public perceptual hash database (http://www.phash.org/) for this kind of stuff.

[rendx]

Such databases exist, but I don't think there's any that is publicly available. https://wiki.openrightsgroup.org/wiki/Indecent_image_identif... lists some, there's more.

[dsl]

There is a government ran database of all known child exploitation images. For obvious reasons you need to show a reasonable need for access. Contact the National Center for Missing and Exploited Children.