People are recommending very exotic solutions of image detection. But unfortunately that's an ongoing arms race between two groups of very smart people. One driven by the moral justification to stop abuse against children and the other driven by their unfathomable lusts.

So I would rather just K.I.S.S. and put your time into maintaining a reporting system where any anonymous user can report violations.

Then put emphasis into making this system as easy as possible for you or any potential moderators. The ideal would be to have an app that would notify you of any new reports and allow you to just swipe from image to image pressing an icon if you want to delete, ban or whatever.

Edit: Such a system could eventually form the basis of a bayesian filtering system.