I think you probably should zap it because MN could be liable in some way for publishing offensive images. The person posting could be possibly be criminally liable for indecently exposing himself. We don't even know if the person in the pictures is the person posting - in a worst case scenario it could be revenge porn.
If it were possible just to remove the photos that would be a good option.
I don't really think you can risk leaving it up no matter how witty the replies because of the risk to the site.