Company Photo Formation?

Hello all. I remember reading somewhere (possibly on this forum even) that in an Army company portrait there are positions in the photo based on MOS. The mortarmen go somewhere, the lieutenants go somewhere, etc. My Uncle Jack O'Hara was a mortarman in I Co, 158th Infantry. He later received a Purple Heart and BSM for Valor. I found his company's portrait on the Arizona Memory website and I asked my mother and grandparents to help identify him in the photo. Their best guess was the gentleman holding the company's flag. If he was a mortarman, would that be the correct position? If not, where would it be in the photo?

Here's the URL:
http://azmemory.azlibrary.gov/cdm/singleitem/collection/ammbush/id/13/rec/9

 

I don't know for sure but my guess is that it is a possibility, if mortar men were placed at the back then that is possible, maybe look up mortar men positions in photos or something. Correct me if I'm wrong, I know nothing about Americans in WWII, I assume mortar men are infantry backup so maybe it make sense them being in the back.