I'm sure it's easy to find "training data" for this project, but FYI 4walled dot cc is probably a GREAT source. It's an archive of thousands of 4chan images, and most are already classified by SFW / Borderline / NSFW. Since it came from 4chan, it includes lots of non-pornographic NSFW things as well (gore, swastikas, etc.), and lots of duplicates with various cropping, reencodimg and other processing done to them.