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.
