This is the only comparable 'left side' photo of Wagner I could find, and it resembles Ben's picture a lot:
Also, from a common-sense perspective, it's unlikely a photographer would have set up on this side for a right-handed hitter, especially Wagner - taking a photograph involved more preparation back then, so the photographer would have wanted to be on the correct side. He was probably expecting Wagner to be batting right-handed and got surprised. Given the 'wrong side' set-up, to me that builds a better case for it being Wagner.