Disable Gravatar for User Profile Avatars in Your Project

Hello! I've noticed that the user profiles in my project are sometimes showing images, seemingly pulled from Gravatar, which feels like an invasion of privacy and we do not want them. How do I prevent Gravatar from being used for user profile avatars? If this is not a setting that can be disabled (I hope it is), and instead we must set an alternative value for $avatar, what value should be provided when we do not want an image used? We prefer to have all user profiles use the colorful anonymous illustrated faces.