How to build a url of a GRAVATAR image from a given email

Use this:

$userMail = whatever_to_get_the_email;

$imageWidth = '150'; //The image size

$imgUrl = 'http://www.gravatar.com/avatar/'.md5($userMail).'fs='.$imageWidth;

You can find a sample script with PHP code on their implementation site: http://en.gravatar.com/site/implement/php