There are people who can "take a picture of you" just by looking at you for a second. They have you memorized after that.

I believe the usual approach is that in general, if you're in a public space, you accept pictures may be taken of you. But it depends on the context. If you're a bystander in your city while tourists are fotographing places of interest for example, and you make it into the picture, then that will hardly be a problem in any practical legislation. Most legislations probably allow for pictures taken of you even without you being asked explicitly, as long as certain rights are not violated.