Gravatar! What is it?
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?
Visit Gravatar site to create one for you. Its easy.
And where ever you post anything with you email there you gravatar would be displayed.
How it works?
A commenter creates an image suitable for a gravatar and uploads it to his account on gravatar.com. The gravatar is rated using pretty much the same ratings as the movie industry: G, PG, R, and X. These ratings are used by bloggers to set limits on the kinds of gravatars that appear on their sites — a gravatar with a G rating will appear everywhere while a gravatar with an X rating may not appear on many blogs at all.
Meanwhile, a blogger (like me) sets up his/her blog to enable it for gravatars. Then, when a commenter submits a comment, the blog’s gravatar plugin (automatic since WordPress 2.5) takes the commenter’s e-mail address (submitted in the comment form) and attempts to find a match at gravatar.com. If it finds a match, it displays the corresponding image. (The e-mail address is not used anywhere in the underlying page code.) If there’s no image on file, the blog software either displays nothing or displays a default image chosen by the blogger.
How the URL is constructed?
Step 1, The URL base: A gravatar is a dynamic image resource that is requested from our server. The request URL is presented here, broken into its segments. The URL always begins with
Step 2, The email hash: The next part of the URL is the hexadecimal MD5 hash of the requested user’s lowercased email address with all whitespace trimmed, for example the fictitious “iHaveAn@email.com” would have a hash of “3b3be63a4c2a439b013787725dfce802” This may be followed by .jpg (useful for some places on the internet which require that image URLs have filename extensions)
Everything from here on… is optional and can be found here