There are a lot of successful people on this site that work in industries/circles that they do not want their identity known for fear of their professional lives suffering. As someone who works in the creative field I doubt any pictures of me dressed up fancy on the internet will cause any damage. However, if you are in a more conservative or political (corporate politics, not government politics) industry, something like talking about clothes all day on the internet could have much more far reaching professional and social ramifications.
Not to mention that there are probably thousands of billable hours a month spent on here. Though I think it's safe to say my boss is not an SFer.
Then there is the obvious privacy associated with not having your face all over the place. I think a mix of all three are good enough to keep most people from posting pics of their face.
Me? I don't post pics of my face because I have a shitty camera and am inherently lazy.