If Aeris was MEANT to appear as a ghost after her death
Then that would make the theory more sense that Zack was MEANT to appear as a ghost in the AC framed photo as well.
But why the hell would he appear after he had effectivly said his goodbyes, along with Aerith, at the end of the movie.
Please feel free to read my take on the official novalisation of Dissidia Final Fantasy at this link: