I am not a lawyer. But generally, there's no reasonable expectation of privacy in a public place. In big cities, you're photographed all the time - surveillance cameras and the like - and you probably don't know it.
If you wind up being a "face in the crowd" in someone else's photo - you're not the focal point, you just happen to be there - there's nothing you can do. And, realistically, if you're in the background of a photo someone took of a celebrity or a cosplayer, or even just a general crowd scene, nobody's looking at you anyway.
But it's another thing to just walk up to someone and take their picture without permission. Legally, you may have the right to do it - again, I'm not a lawyer - but morally, it's rude.