Change default gravatar image from your wordpress blog without any plugin!!

When we create an account a wordpress blog then we see a default gravatar image like this:

(although that gravatar image you can change.) This gravatar image choosen by wordpress team. but if you can want, you can change this default gravatar image without plugin. when a new user will register your site then him/her default gravatar will changed automatically. for this please follow:
1. Copy this codes:

add_filter( 'avatar_defaults', 'custom_avatar' );
    function custom_avatar($avatar_defaults){
    $custom_avatar = get_stylesheet_directory_uri() . '/images/author-site.png';
    $avatar_defaults[$custom_avatar] = "My Default Avatar";
    return $avatar_defaults;
 }

Any problems copy code here:
2. Now paste it on yor wordpress themes 'functions.php' file and save it.
Done!!! now you can change default gravatar from settings like this:

Thanks for visiting my post.