Posting a link to your images in your signature and a photo of yourself as your avatar make one narcissistic and an egomaniac?? Guess I am then.
We post here because we're into photography. It makes sense to me to share links to our photos so we can see what each other does. The avatar photos are way too small to share any meaningful photography and putting our photo there gives the reader a bit of a visual of who he is interacting with, that's all. Of course little icons relevant to our interests or sense of humor or whatever are common too, anything that makes a bit of a visual personal impression.
Seems pretty normal to me.