I know little about mirror neurons other than that they exist and that they play an important role in socialization, but I would suppose those of a narcissist would react in measurably different ways to images of themselves relative to anyone else.
It’s highly unlikely this has not been proposed before, but I think this is the first time I have proposed it.
I would add: what is a selfie but an affirmation you exist, at least in the “eyes” of a camera you can control?
Further question: do they arise from narcissism, build it, both or neither? How important a role do they play in the mental health of those who take a lot of them? Do they help, hurt, or a bit of both?