After you get your blog started and folks start commenting, you will see small “icons” next to each comment. These are avatars that can represent you or any other person who writes or comments on your blog.
WordPress supports the Globally Recognized Avatar or gravatar service. Basically, it is an image that follows you from site to site appearing beside your name when you do things like comment or post on a blog.
Gravatars are based on images you upload and are associated with an email address. So, you could have one gravatar associated with a professional email address for work or school and a different one associated with your personal email address (or more).
It is fast and easy. Step one: Go to the Gravatar site and get yourself set up!
Need more explanation, check this out: