It’s been awhile, but I believe you have to go to gravatar.com (which hosts the avatars for all WordPress accounts), log in with your WordPress info, and then you can update your avatar image.

It’s based on the email address in your Contact Info in your profile here. You’ll need to create a separate Gravatar account and then add that email address to it (they’ll send you an email to confirm it). Also it takes a bit to update, depending on how things are being cached.