I have just configured this blog to display gravatars next to each comment. A gravatar is a globally recognized avatar. You can sign up for free on gravatar.com, and upload a picture that you want associated with your email.
Then when a web site wants to display an image associated with your email address, it encrypts the email address with md5, and then sends a http request to their web site. Since it is using md5, it will not be possible to determine the email address from the url.