goto your settings > Discussion > scroll to the bottom and make sure “show avatars” is checked. Select the appropriate rating for your site then select what logo to show if the user doesn’t not have a gravatr set up. Default is the blank man I believe. Then follow the instructions in the link below and it will tell you how to set a gravatar to your user.
Whatever email is registered on your site must be the same email used on the gravatar site, although you can add multiple emails to one gravatar account.