How to Change the Default Gravatar on WordPress

//***Custom Gravatar
Main Post: http://www.pchelpcenterbd.com/wordpress-tips-12801
**/

function pchelpcenter_custom_gravatar ($avatar_defaults) {
     $myavatar = get_stylesheet_directory_uri() . '/images/defaultgravatar.png';
     $avatar_defaults[$myavatar] = "PC HELP CENTER (BD)";
     return $avatar_defaults;
}
add_filter( 'avatar_defaults', 'pchelpcenter_custom_gravatar' );