One thing to mention is that summoning something as a primal tends to change its appearance somewhat. Tiamat pointed out that the Bahamut she summoned was not the same Bahamut she'd loved. Shiva too looks nothing like the real mortal Shiva that Hraesvelgr loved. It's possible the Bahamut we know looks nothing like the original so comparing his images in-game to the art (which presumably depicts the seven children before everything fell apart) might be a futile effort.