WP-Mix

Using your own custom gravatars (aka avatars) in WordPress is a piece of cake.

Just add the following snippet to your theme’s functions.php file:

function display_gravatar($avatar_defaults) {
	$myavatar = get_bloginfo('template_directory') . '/images/custom-gravatar.png';
	$avatar_defaults[$myavatar] = 'My custom gravatar';
	return $avatar_defaults;
}
add_filter('avatar_defaults', 'display_gravatar');

Make sure you upload your custom gravatar image to the indicated directory and edit “My custom gravatar” to something that meets your needs.