Assuming you're on the system they are doing the tagging on (it's easy when it's just Facebook, but a photo could be tagged on Flickr with your real name and you'd never know), and you check your email (or whatever method of notification they provide), and a host of other things that put more work on those who are trying to control their exposure. That's the problem with opt-out, and that's why thinking you can maintain your privacy by monitoring what other people do is folly.

If the image tag on Facebook isn't linked to your profile, there's no way for anyone to find it, unless they were friends with the poster and browsed to the photo.

Flickr is more public, but they don't do any user-graph linking at all, the photo annotations are plain text. I don't think the annotations are even exposed to google.