How to enable gravatars in WordPress?
Gravatar stands for Globally Recognized Avatar. Users can set their avatars at http://gravatar.com/ and they will be displayed alongside their comments in any website that supports them.
Starting from WordPress 2.5, Gravatars are built-in and don’t require any additional plugins for basic usage and management. You can manage your Gravatar from the Settings > Discussion section of the WordPress admin panel.