To make the avatar show on your comments and user/author profile you'll need to make sure that you have an account on http://en.gravatar.com/ with the same email address as your blog email - and make sure to upload your picture there as well.
Once you do that, your gravatar should start showing on your blog posts.
A Gravatar is a Globally Recognized Avatar. You upload it and create your profile just once, and then when you participate in any Gravatar-enabled site, your Gravatar image will automatically follow you there. Gravatar is a free service for site owners, developers, and users.