Is it correct to say like this?
John: Which one is you in the photo? (correct or not?)
Mary: The middle one
If you are both looking at the photo at the same time, there's no need to say "in the photo".
1. Which one is you? — The one in the middle.
But you might be more likely to hear:
2. Which one is you? — That's me in the middle.
Not a professional ESL teacher.