WP Admin thumbnails

Hey

When you upload a picture to the media library, WP generates a thumbnail (size set by you) that itself uses when you look inside your media library.

Is there a way to extract that? I am using a catch-that-first-image-in-the-post function but I dont know PHP that well and I wanted to know if there is a better way to do it.

The function I use is:

function catch_that_image() {
  global $post, $posts;
  $first_img = '';
  $nothumb = get_bloginfo('template_directory');
  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
    $first_img = "$nothumb/images/default.jpg";
  }
  return $first_img;
}

Can it be changed to load the thumbnail of the first image instead?
Thanks in advance.