If you look at it as a form of revenge pornography where the intention of publicly sharing it is to demean and shame the victim the harm should be pretty obvious. People kill themselves over selfish actions like that.
I believe that the parent comment's point is that if the default assumption that a nude of a real person on the internet is fake, the nude should lose a lot of its power to shame. If nobody believes that a picture is real, nobody should believe that the events in the picture happened.
I'm not sure that's true, though. An evil rumour should be assumed to be untrue, yet they're often believed and often cause significant harm.
People have a tendency to keep ahold of their first opinions. And the first opinion will not be "this is obviously fake" - because it looks real, and we're animals first, humans second.
It's not really the same, rumors are more privileged. Rumor is a method of distribution of information and you can't dispose of distribution of information.