[true_religion]

I hope I'm not reiterating anything people have said... but generally sites with a community focus, have volunteer moderators who monitor for this stuff and help ban users who post it.

If your'e doing anonymous posting, and the images are meant for sharing on other sites (ala Imgur) then you'll have a harder time of it. You can ban the images themselves and save the hash of the image to compare to future uploads and automatically reject those.

I can probably tell you more about how we do this on my own sites and my newer project, 9Cloud.us, which allows site owners to outsource all these concern. Just email at [e-mail]