Use one of the OpenID options to log in. Have an avatar on the service you select, and DocType will find it.
UPDATE I think Justin Hileman's answer is better than mine. I just assumed DocType was grabbing my avatar from my Google profile, as I never uploaded one here. But on closer inspection it does seem to be coming from the Gravatar service.
If you use a password type login, Doctype will automatically fetch the Gravatar mapped to the email address you use to log in. Go to gravatar.com and sign up to upload an avatar.
Note: My browser was caching the default avatar, so it took a force refresh for me to see my pretty little mug instead of an anonymous silhouette.
Excellent, thank you very much.