Child sexual abuse imagery is absolutely rampant.
NYT: Child Sex Abuse offers some insight, I listened to a podcast based from the same journalism. Unique and specific key words can be typed into google or bing right now, and will instantly bring up images. It is easy to imagine Reddit being used to facilitate the exchange of such words, codes, naming schemes, and occasionally images by brazen pedos.
The story in the podcast had an interview with a mother who divorced after she discovered her husband to be uploading pictures of their daughter. Part of the tracking by the government organizations include notifying the parents when the report media pops up again. But once the girl turns a certain age (I think it was 18), she will automatically receive those notifications. Problem for the mother is: her daughter doesn't know because she was so young, and the mother has protected her since.