I am quite sure that people who post erotic material online know exactly what people could do to their pictures. I mean, what is the purpose of sharing those pictures at all if not to let other people admire/enjoy their work? I find your post very naive to be honest. If you post something online free for all to see on the internet, how do you even consider it private material??
If you have pictures that are considered erotic and you do not want others to enjoy it, you do not post it online to be viewed by everyone. I think what you call "not the point" is exactly the point of posting it.