Ha! Imgur is awash with pornography. They could automatically remove a large amount of nudity and porn by looking at the contents of /r/nsfw. As a second gate, they could use Microsoft's image classification service, which has a 'racy' flag.
Imgur is lucky that there aren't powerful rightsholder bodies for images the way there are for music and video. That's the only way the massive copyright infringement happening there every day goes unchallenged. Sure, they have DMCA safe harbor, but so did many other video and music startups who ended up slapped with lawsuits.