Recently I start using Gravatar (an abbreviation for globally recognized avatar). It is a free service for providing globally-unique avatars which was created by Tom Werner for blogs and web forums. I want to embedded it in my WordPress 2.7 template. So I try to insert the additional code as below in comments.php and it works. I set my icon size to 50x50. Others might like 40x40. It's depend on personal preference.