写真の左から~人目 is unambiguous, but "a person in the photo の左" is ambiguous.
e.g.
青いシャツを着た人の左にいるのが父です。
The two interpretations; "from the viewpoint of the one wearing a blue shirt" and "from the viewpoint of the speaker" are both possible in this sentence. In this case, 向かって is used to make clear which viewpoint the speaker is referring to.
青いシャツを着た人の、向かって左にいるのが父です。