/**
  * get_author_image_url()
  *
  * @param int $author_id
  * @return string $image
  */
 static function get_author_image_url($author_id = null)
 {
     if (!$author_id) {
         $author_id = author_image::get_author_id();
         if (!$author_id) {
             return "";
         }
     }
     $author_image = get_user_meta($author_id, 'author_image', true);
     if ($author_image === '' || $author_image == '0') {
         $author_image = author_image::get_meta($author_id);
     }
     if (!$author_image) {
         return "";
     }
     $author_image = content_url() . '/authors/' . str_replace(' ', rawurlencode(' '), $author_image);
     return $author_image;
 }