Fortunately this is an easy thing for Imgur to solve in 2017 if they really want to with machine learning. Enough is available online that almost anyone could do it at this point.

Indeed, this is a solved problem. See: Not Hotdog.

You don't even need to do that.

Just have people opt in for NSFW content on their accounts. Leave them opted out by default (or public).

There is no reason it shouldn't be a human controlled flag.

There is a Mature filter on imgur already that's human-determined. The problem is that people posting pictures will mark things mature unnecessarily, and there have been trolls who put up hardcore pornography without marking the image as mature. The problem is, as always, trusting humans to do the right thing without massive consequence for doing the wrong thing.

I agree that should be the default but getting the data to do it automatically is hard if you're going to ask users to mark things NSFW on their own accord.