Readers of comments on the Stubborn Mule and other blogs may have noticed the little avatars like the one pictured here. Some may even have wondered how it is that some commenters manage to display a picture of themselves. If you are one of those people, or you are now curious, read on.

These avatars are known as “gravatars”, or globally recognized avatars. Gravatars provide a clever mechanism for frequent blog vistors to have the same image appear with their comments across all gravatar-enabled blogs. To create a gravatar of your own, you simply sign up at and upload an image.

Most blogs require commenters to provide a name and an email address. Some people are reluctant to provide a legitimate email address, but there are advantages. Email addresses are not published on the blog, but they are available to the administrator of the blog and may be used, for example, to notify you when new comments appear on the blog post. Email addresses also form the basis of the gravatar system. Each gravatar image you upload is associated with an email address. Whenever the blogging system displays a comment it will request an image from If there is a gravatar associated with the commenter’s address, it is sent to the blog. Otherwise, a randomly generated image will be returned. For example, here are the gravatars for my personal email address and the mule’s email address.

Currently, there are three different types of random gravatars: identicons, monsterids and wavatars:

The mechanism associating email addresses and gravatars is cryptographically secure to ensure that visitors to the blog cannot reverse engineer the email address from the image or its address. The way it works is that a MD5 hash is made of the email address and then used to form the gravatar’s url. For example, the hash of the mule’s email address is 60c746b19b5c687bf9029d3da8a7c27f and so the url of the mule image above is

If you are interested in testing the email addresses of people you know, you can use an online MD5 hash generator to determine the hash and use it in place of the mule’s hash in the address above.

So, now all you have to do is create a gravatar and test it out by commenting on the Mule!

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