I agree with Dan Benjamin on why your avatar matters:
A great avatar will help people remember you instantly. And you should use it everywhere, across the board. Use it on your blog if you have one (you should). Use it on Twitter, Facebook, LinkedIn, everywhere. Even if people don’t automatically remember your name or your website, they’ll remember your avatar and make an association.
I try to use the same image of myself everywhere – but it’s almost 5 years old now so I guess I need a personal rebrand.