I think because there was a flashback in a TNG episode that recounts when Picard was impaled as a cadet, I believe. He had at least some hair, and darker. Even in early episodes of TNG Picard had more hair than the completely bald Picard in the NEM photo.Why? Many people go through multiple hairstyles in life.
It wouldn't have been a problem to add that bit of hair to Tom Hardy for the photo. Would have made things more believable and could have done a little more for the overall lazy attention to some details in NEM.