I also agree that he needs to stay dead (as I also believed Sirius needed to stay dead). I don't think he'd be a portrait if he weren't really dead, and I do not subscribe to the Dumbledore-as-a-phoenix theory at all. The phoenix might come back, but I think Dumbledore's participation is/should be the portrait, and in a limited function, at that. Harry needs to do this on his own.