Last night, I added gravatars to the comments on this site. The best blog post to see them is Blog Tipping.

Gravatars are globally recognized avatars. It's a clever way to let people upload a little picture that goes beside their comments. This way, people only have to add them once (at the Gravatar website) and they automatically work across all web sites implementing them. They use the MD5 hash of your email address as an id, so you only need to put in the same email address when you write comments, and your gravatar will come.

Implementation is very easy. You don't need to make any HTTP request on the server or anything. You only need to add an image that points at the gravatar URL, with some optional parameters, and the browsers of people visiting your site will download the images from gravatar.com.

The only required parameter is gravatar_id, the md5 hash of the email address. Every programming language has a way of finding an MD5 hash. PHP probably has the most simple example:

<img src="http://www.gravatar.com/avatar.php?gravatar_id="/>

There are also optional parameters you can use. rating lets you make sure there are no pornographic or mature images on your site. size lets you set the width/height to something other than 80px. default lets you set the URL of a default image, in case you want to display something for people without a gravatar (a 1x1 invisible image is the default). border is supposed to let you set the colour of a 1px border on the image, though I wasn't able to get this working. So the output of a complex example might look like this:

<img src="http://www.gravatar.com/avatar.php?size=50&border=000&gravatar_id=eb22390416c3b62025dc9ad2120a8ade"/>

It's worth noting that adding gravatars can be a css and design challenge. You don't know whether the image will be a real gravatar or just a 1x1 invisible image. If you add margins or padding to the gravatar, they will end up on the invisible pixel as well, and this can look weird. I solved it by indenting all comments 60px to allow space for a 50px image. If the image isn't there, the comment is just indented anyway. You can also solve this by setting a default image to a "No gravatar" image.

Gravatars can really be implemented anywhere you know an email address and may want to show an image. There is a Thunderbird add-on that lets you see Gravatars while you're reading emails.

For more details, or to see how to implement gravatars in other languages and blog publishing software, the Gravatar web site has a detailed implementation guide. If you just want to upload your own gravatar, go sign up.