Gravatar is a very helpful service. Instead of uploading my profile image in the many websites I have registered, I only need to upload my avatar to a single place where every website could easily access to it. Also, if you register to a new website, it will also use your Gravatar profile image directly without any effort from your end.
Great service but is not under a free software license and cannot be hosted on your server.
This is where Libravatar differ.
Libravatar is a free (APGL) and opensource software and as Gravatar, is an online avatar hosting service.
Their belief is that centralised approaches don’t put users in control, so they let you now also control how your profile pictures are served to others websites.
For example, if you run your own business, you may want to integrate Libravatar with your existing systems (Staff directory, …) and delegate the authentication to LDAP. Then you could choose what happens when email addresses or profile pictures are not found, for example you could display your company logo instead.
The installation is not easy but all the steps can be found here.