It is a self-portrait, but not in the way you think. The picture is himself, you and your wife all as a single entity. That entity is him. He is as much you as he is your wife as he is himself, and he views himself this way. The three of you are extensions of each other, not truly separate beings; he is showing you that who he is is mother-father-him: all one, him. He is telling you that he loves you. That's what it means. He wants you to know he loves you.