This is why the social web is a mess. To be honest, I haven't read, for example, Facebook's use agreement, so far all I know, in my ignorance, I probably gave permission to some far off porn site to use my picture because some friend of a friend of a friend on Facebook posted a comment on a porn site somewhere.
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 If a porn site starts using your picture, you can serve them with a notice notifying them that your permission has been revoked.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).