The image that appears in your User Profile widget is automatically drawn from your Gravatar.com account. If you do not have a Gravatar.com account, you can sign up for one here. This image will appear anywhere on the internet where your corresponding email is used to comment.
From Gravatar.com "Your Gravatar is an image that follows you from site to site appearing beside your name when you do things like comment or post on a blog. Avatars help identify your posts on blogs and web forums, so why not on any site?"