//Make a new default gravatar available on the dashboard
	  function newgravatar ($avatar_defaults) {
	    $myavatar = get_bloginfo('template_directory') . '/images/tweaker.jpg';
	    $avatar_defaults[$myavatar] = "Tweaker";
	  return $avatar_defaults;
	}
	add_filter( 'avatar_defaults', 'newgravatar' );

Customize Default Avatar 

wordpress

PHP