Message parts are not templated (as they are rather complex), so the customization process is more involved. I would do it this way:
1. create a new MessageList.ascx in your project with the accompanying codebehind file and inherit from the MonoSoftware.MonoX.ModuleGallery.SocialNetworking.MessageList
2. do the same for the MessageCenter.ascx and change the line that references the original MessageList to point to your new MessageList control
3. create a new GridTemplateColum in the grid that is used to display messages that will contain a Gravatar control

That should be it - place your custom MessageCenter part on the page and it should contain a MessageList with the user avatar.