This is a bit complicated: what if you had some sort of capcha that required users to classify images as nsfw/sfw/illegal?
If the user normally encounters photos on the site by requesting them (e.g., by entering a search query or browsing a friend's album) rather than having random photos thrown in their face (like HotOrNot.com), I would think you could run into some very upset users (and possibly legal problems) if you are throwing random photos that might contain disturbing images in their faces. I mean, if you go to a website intending to browse photos your friend took of his boat and the site throws up some random child porn on your screen, you'd be pretty annoyed, right?
Forcing your users to look through a selection of random images which could include several which may be illegal for them to view is not okay at all. Especially if you know you're putting them at that risk!
This would indeed be a problem at the start. But after some time it could weed out. People wanting to share illegal images would go away from the site if they ger rejected. We could expect a significant drop in nasty images.
You would to expect a significant drop in traffic after the ensuing PR nightmares, fbi raids. Followed by years of lawsuits from people who claim PTSD symptoms and what not after being forced to watch CP.
If the user normally encounters photos on the site by requesting them (e.g., by entering a search query or browsing a friend's album) rather than having random photos thrown in their face (like HotOrNot.com), I would think you could run into some very upset users (and possibly legal problems) if you are throwing random photos that might contain disturbing images in their faces. I mean, if you go to a website intending to browse photos your friend took of his boat and the site throws up some random child porn on your screen, you'd be pretty annoyed, right?