I've seen several people ask about this in the forums - How can I use my own picture instead of the boilerplate Grok picture in my posts? - and I thought I'd start a separate topic to answer the question.
MDA uses a service called Gravatar. Just go to http://en.gravatar.com/site/signup, create an account, and upload your picture. Once this is done, that same avatar will automagically show up with your posts on lots of different social networking sites, including MDA.
Kudos to the MDA team for using Gravatar. I love that I didn't need to make Yet Another Avatar for myself here - my Gravatar was already in place and showed up on my very first post.