Diablos is the most obvious. I always thought it was him. I dont think it would be Selphie because she wasnt actually a messager she was only told to deliver one message. Just like when your in school and the teacher says, 'you, go get such and such from mr whatevers classroom'. This person wasnt the class messenger, they were picked randomly. Usually because they sat closest to the door. But anyway, as i said before, it points mostly to Diablos. If that p[ainting does actually mean something, then the other paintings must also. I always thought the Judgement painting was Alexander.