This is my opinion. That picture that you see the 'ghost of Zack' in (between Barrett and Cloud), I think is not him. If you look on the opposite side of the picture, there is a rectangular section of the stone roof that is identical, 'face' and all. They are merely texturized structures of the building. I just feel that if there was any intention of putting a ghost of Zack in this particular spot, it would have been made more obvious. Especially since it's a moving picture and it's harder for the audience to spot these things.