This tutorial is going to show you how to change the default Mystery Man gravatar with your own gravatar image:
1) You need to visit http://www.gravatar.com and register using the email address associated with your WordPress admin user account, i.e. the one you have in your WordPress admin under Users -> Your Profile -> E-mail (required).
2) In order to register, go to Log in -> Sign up from the top menu or click on "Get your Gravatar today"
3) Sign up on the next page using your WordPress admin login e-mail address
4) Check this mail box. Open the e-mail that is going to come and click on a link in it to confirm your registration.
5) On the next page you are going to fill all fields in and click on "Signup"
6) Then visit Your Public Profile and click on the link to Change your Gravatar.
7) On the next page you need to choose one of the options
suggested to upload your gravatar image
8) Follow the on-screen instructions to set a new avatar for your account.
9) After you’ve done that your gravatar.com image will appear on your blog instead of the "Mystery Man" image in 5-10 minutes. You need to clear your browser’s cache to see the change.
Please, also watch the demo movie below