Replace the Gravatar by QQ avatar (if the author uses QQ mail) for those who do not have a Gravatar, and cache all the avatars.
- Place "default.jpg" under the path: ABSPATH . 'avatar/' (This is also the location of the cache).
- Add the content between "<?php" and "?>" of avatar_wget.php to "functions.php" of your theme.
Please make sure that exec() is enabled, and wget is installed.