Components->User Enhancemennt Manager
Click the component options button at the top right
Set show gravatar to yes.
Save & close
Click on custom user profile fields
Press rebuild button in toolbar
You should now see the gravatar when you edit a user in user manager, assuming they have set up a gravatar image associated with their email address.
If you do not have a gravatar image, you can sign up for a free one via http://gravatar.com/
The gravatar image will also appear on the front end user profile.
For users who have not associated a gravatar account with their email address, you will see a blue gravatar logo.
For the gravatar feature to work, you must have enabled the "User - UE Man".
The "User - UE Man" should have been automatically installed and enabled when you installed User Enhancement Manager.
There are no configuration options for this plugin, just make sure it's enabled.
The User Enhancement Manager component dynamically builds the user plugin. If you happen to use a tool that checks for modified files on your site, note that it is normal for /plugins/user/ ueman/ueman.php and /plugins/user/ueman/profiles/profile.xml to be modified.
Also if you have caching or CDN turned on for your site, you may not immediately see your changes, so it is recommended to turn these off whilst configuring this extension.