Here’s a cool tidbit from the documentation for Moodle 2.2, there will be a setting to check (if the user has not already uploaded an image) a user’s email against Gravatar and will use that image instead of the default.
Gravatar is “globally recognized avatars” and are used throughout popular sites like WordPress. By uploading one image to your Gravatar account it’s used throughout the Gravatar network as you move across sites and join new networks. It’s easy to use and is a nice additional feature of Moodle 2.2.
Read more at http://docs.moodle.org/20/en/User_pictures