I'm fairly certain that in the Cosmology picture that the flowers the Oracle (woman in white) is holding represents Gentiana as a guiding force for the Oracle. The Astrals had a hand in creating the world, and the Gods gave Lucis the crystal. This is what's being depicted partially on the front of this.
As for people speculating that Ardyn is Bahamut, perhaps, but it doesn't change the fact that the Bahamut like figure is giving the King the crystal. Look at his hands, they are on the outside of the king's hands. When you clasp your hands around someone else's like this it implies that you are giving them something and you are telling them to hold onto it. Also, look at the eyes of both these two. The King looks at the crystal in awe, while the Bahamut like figure looks into the kings eyes. This also shows how this Bahamut like figure is giving the King the crystal, and that he's focused on the King's reaction, while the king is enamored by the crystal.
How Ardyn fits into this, I don't really know, perhaps he doesn't. As people mention though, the hairstyle sort of is similar.