Claws Mail, my favorite e-mail client, is able to display avatar images using the Libravatar protocol.
Ricardo Mones of the Claws Mail Team wrote a libravatar plugin that fetches the avatar image for the email sender and displays it on the top right of the message pane:
The plugin does not yet support federation, but I hope that it will be implemented in the near future.
The next version of Claws Mail - 3.9.4 - will ship the plugin.