/**
 * Customize Default Gravatars
 * http://digwp.com/2010/04/wordpress-custom-functions-php-template-part-2/
 *
 **/
function myfunction_custom_gravatars($avatar_defaults) {

	// change the default gravatar
	$customGravatar1 = get_bloginfo('template_directory').'/images/gravatar-01.png';
	$avatar_defaults[$customGravatar1] = 'Default';

	// add a custom user gravatar
	$customGravatar2 = get_bloginfo('template_directory').'/images/gravatar-02.png';
	$avatar_defaults[$customGravatar2] = 'Custom Gravatar';

	// add another custom gravatar
	$customGravatar3 = get_bloginfo('template_directory').'/images/gravatar-03.png';
	$avatar_defaults[$customGravatar3] = 'Custom gravatar';

	return $avatar_defaults;
}
add_filter('avatar_defaults', 'myfunction_custom_gravatars');

http://digwp.com/2010/04/wordpress-custom-functions-php-template-part-2/

Wordpress: Customize Default Gravatars

wordpress

PHP