¿Plugin WP para asignar automáticamente imagen destacada?

function catch_that_image() {
  global $post, $posts;
  $first_img = '';
  ob_start();
  ob_end_clean();
  $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
  $first_img = $matches [1] [0];

  if(empty($first_img)){ //Defines a default image
    $first_img = "/images/default.jpg";
  }
  return $first_img;
}

<img src="<?php echo catch_that_image() ?>">

//primera imagen como destacadafunction auto_post_thumbnail() {
          global $post;
          $already_has_thumb = has_post_thumbnail($post->ID);
              if (!$already_has_thumb)  {
              $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
                          if ($attached_image) {
                                foreach ($attached_image as $attachment_id => $attachment) {
                                set_post_thumbnail($post->ID, $attachment_id);
                                }
                           } else {
                                set_post_thumbnail($post->ID, 'ID_imagen_por_defecto');
                           }
                        }
      }
 //Ahora añadimos la función al loop
add_action('the_post', 'auto_post_thumbnail');
add_action('save_post', 'auto_post_thumbnail');
add_action('draft_to_publish', 'auto_post_thumbnail');
add_action('new_to_publish', 'auto_post_thumbnail');
add_action('pending_to_publish', 'auto_post_thumbnail');
add_action('future_to_publish', 'auto_post_thumbnail');