associated template variable
Specify the full URL of an image to display when an email address is not associated with a gravatar.
Gravatars are generated from email addresses, and they appear only for folks who have registered with gravatar.com and uploaded an image (avatar) for themselves.
Since not everyone has a gravatar, sites have the option of specifying an alternate image to display.
If you leave this entry blank, the Gravatar.com site will return a transparent 1x1 pixel gif image for email addresses that do not have an associated gravatar.
And, of course I should mention that your comment form must collect an email address from visitors if you want to use gravatars.