Your guess is not bad, but it in conversational English, it would be better to ask “which one is you?” The verb “is” will can agree with the subject (“which”), or with the predicate nominative (“you”); it doesn't really matter.

firstly, it should be written Who are you?

No, 'who are you' is incorrect. You need to ask, 'Who's the woman in the photo ?' or a variation of this like: "I'm uncertain of the woman in the photo. Can you tell me who she is ?"

My friends sent me a picture. But I cannot figure out who the woman in the photo is. How can I ask? Who are you?