It's a valid theory to study. Is there a link between Narcissism (or other mental illnesses such as depression, which could account for a need to gain approval from others. Psychopathy too) and selfies? I can't make that determination yet, but I do know that Narcissist are prone to acts of vanity, maybe there is a link?
I still wouldn't classify all "selfie-ists" as Narcissistic though. Not everyone is taking self-Eportraits to gain other's approval, some just do it for fun or to share their experiences with people they know.
Those that share them with people they don't know, or are doing so for approval only are the ones I'd look at more closely.