Currently, there's two ways to have a custom avatar on the site.
If you used Gravatar (http://en.gravatar.com/
) before, just make sure that your email address that's registered here is linked to your account. If not, you can register and set it up pretty quickly.
I'm currently working on a "Edit Profile" section where you'll be able to upload your own images to use as an avatar (as well as edit other parts of your profile), though in the meantime, I can upload it to your account for you if you don't want to use Gravatar.