You can retrospectively retract your consent at any time. If a porn site starts using your picture, you can serve them with a notice notifying them that your permission has been revoked. If they then continue to use it, you can seek civil or criminal redress through the courts (depending on your / their jurisdiction).
Well, in that case .. I better start hitting some of these porn sites for "research" to make sure I'm not showing up anywhere. I better check every pic. I have a responsibility, after all