Trust me when I say I know narcissism, as I have a parent who has been one their entire life. For the majority of people posting, or posing, could indeed be a way of looking for positive reinforcement, feeling good about themselves, etc. Or, in some ways, as could be the case of an online community, engaging with the community and interacting. However to unilaterally call it narcissism is not only unfair, it's likely highly inaccurate.
The definition of a narcissist is someone who believes they are "special and superior". One can't assume anyone/everyone who posts selfies of themselves online does so because they consider themselves superior, or even special. Could it make them feel that way with positive comments? Suppose so, and what's the harm in that? Even a person with a really healthy ego can fall WAY short of being a narcissist.
And the only reason I'm offering this up is because you've insisted twice that you know what a narcissist is, when I don't think you do.
No, narcissist's DO NOT believe they are "special and superior", which is why they create a facade to get others to think that they are. They actually have a poor self-image. That is what causes them to need the constant attention and validation. Your statement is incorrect and a common misunderstanding of the mental disorder.
and please keep posting.