As for Ultimecia's knowledge of Diablos, the fact that she's from the future could explain how she knows about him. Any historical information that survives to her time could be used.
Yes,and why She bother about Diablos anyway.So far Diablos has nothing to do with Ultimecia.So why that picture has to be Diablos.

Unfortunately, before that statement can be accepted, you must first prove that the pictures have some meaning to Ultimecia. You have not done that. You have presented a possible interpretation of how they might be, but you have not given us proof.
Quote Originally Posted by Viator
How can we find any evidence about those paintings?
All we see just beautiful pictures with mystery and of course only Ultimecia can give the evidence about Her paintings.
I cant give you proof because there's no proof in the game that has a connection with the painting in-game info.I never ask you to accept that.

No, with what you have presented us here, your theory cannot be accepted. Certainly, we can not disprove it, but that doesn't matter. In order to have an acceptable theory, you have to prove that it makes sense in the game world.
I think My theory make senses with the game world and Ultimecia.

Well you dont need to believe it.
I answer that mystery just like anybody else before and My answer is not bad,I guess.