You don't transfer your copyright ownership to a site when uploading, it's generally just umbrella licensing terms, not taking away your copyright ownership. Facebook for example, etc.
In any case, if you're paranoid, you could always legally sign over the copyright ownership to a legal entity you create, then license the images to yourself for usage (non-transferable license) (or even don't license, since the legal entity you create wont sue you).
You can then cannot legally license them yourself, only the entity can (which of course you control).
Similar to when people upload and share images from TV shows, celebs, or etc, they certainly don't own the copyright and cannot license it's usage to Facebook and others.