I have no idea what subreddits are using these days, but I'm familiar with spambot, that compared any submitted image against blacklists. For anyone managing a NSFW subreddit, such a tool is an absolute necessity to avoid getting sent to the penalty box by Reddit admins.
https://github.com/4pr0n/spambot
I don't know what's being used now, but when I was familiar with it, spambot was monitoring dozens of NSFW subreddits that had tens of thousands of subscribers. This is only one of thousands of such customized moderation bots that use the free API.