I'm working on a forum that allows images attached to the posts and doesn't require registration. Thing is, I'd like to provide a work-safe navigation option in which the posts with porn images attached aren't shown.

The ideas I've come up with are:

• Making the work-safe option the default and treating all posts with images attached as pornographic, and making them visible only if the user "unchecks" it.

• Making all posts with images attached not work-safe by default and
  changing their status to work-safe only after a moderator approved
  it. Only then they would be visible if the user has the "work-safe" option checked.

Does anyone else have an idea? Also, how the big web services deal with this? (YouTube, CraigsList, even StackExchange).

By the way, I don't think that "nudity detector" libraries are accurate and they give plenty of false positives and negatives.

Thanks!