They both have the Buster Sword because they both owned it, at various points in time. Zack's always been important; we just never knew how much until Crisis Core. And this is an Advent Children pic, so there shouldn't be anybody else from Crisis Core.. although Cait Sith missing is an issue. Overall, though, it's an awesome pic.
The problem with Masamune here is that it it supposed to be a single-edged, curved blade, with a curved tip. Sephiroth's Masamune is an outrageously long katana (or perhaps better labeled as a nodachi).
Voyevoda said: Since when is Zack important enough to draw?
Zack did all the work, Cloud was in the right place at the right time, killed Sephiroth once. Then Cloud combined himself with Zack's personality due to mako poisoning etc. probably had sex with Zack's girlfriend. Rekilled Sephiroth again, this time with help of the other charaters in your party.
Oh right killed him a third time for a movie. (I consider him/Kadaj same person.
The worst part was how Cloud responded to Aerith's death when he was going to kill her himself under the influence of Sephiroth. Later being all emo later.