This is likely the same image hashing they use to detect child pornography, and there's been a lot of work invested into making sure that those types of filters

This is likely the same image hashing they use to detect child pornography, and there's been a lot of work invested into making sure that those types of filters don't prevent the detection. You can very effectively require that the "real" image is visually entirely dissimilar. Now, obviously you can't prevent the actual data from being distributed. For example, you could base64 encode the image and send it as a series of messages instead. The motivation for revenge porn is to hurt the other person. One vector for this is to post it to all of your mutual friends so that the victim is shamed. This adds a barrier so that the mutual friends have to invest work into getting and distributing the porn, and won't accidentally see it flowing through their feed.