I'm not the same person I am out in the real world when I post online. Yes my ideals and opinions remain the same but I express them much more freely in a forum setting. It's a base idea of sociology, we have different persona to fit different environments, situations, and interactions with different people. For instance do you use the same vocabulary and tone of voice when speaking with your parents than when you are around your friends; how bout when on a job interview or ordering food at a restaurant?
Of course everyone is different and if you're comfortable with presenting the same persona across a large array of situations then more power to you, a wealth of confidence is not a bad thing.
As for why I don't post a picture of my self as my avatar? Well that kinda goes back to what I just got done explaining. I feel that I act differently enough when participating in a forum environment that I want to use an avatar fits me in some way, of course the one I use here I use simply because I thought the geico stack of money with googly eyes was freaking awesome. Over time though I like to think that image is how people identify me on the forums. When it comes to the Escapist forums that's the persona I choose to wear and be known by; it's never been a matter of anonymity for me its merely the persona I've taken on.