Doesn't work if someone else violates your privacy in the first place by posting it publicly. To take it to the extreme, someone could break into your house (where you presumably do have a reasonable expectation of privacy), take photos of you and upload them publicly.
Well yes, but if you're assuming that, then that other person who violates your privacy could also choose to allow the photos to be used for AI training even if there is an opt-in or -out.