This is just an idea, since I never built such filter, but you could automate a large part of filtering NSFW images. A quick search on google lead to this paper : http://cs229.stanford.edu/proj2005/HabisKrsmanovic-ExplicitI... Once you have that in place, I guess it's better to make it agressive and report false positive as NSFW.

Google "safe image search" has the additional help of searching the content of the page the image is used. You might be able to do the same, up to some limit, by checking the http referer header field to know where requests are coming from. You could scan the referer's page for some keywords. This might give you a better idea of the context where the image is used. Note that this might be tricky, since you probably don't want traffic coming out of your server to some child porn site.

That said, those are just some ideas. Youtube has a good community that flags videos, but also an army of reviewer that look at the flagged content.

http://mobile.nytimes.com/2010/07/19/technology/19screen.htm...

Another way to look at it would be to try to manually select some images as "front page worthy", instead of trying to filter the bad stuff.