remote picture URL

No add this plugin pictures WordPress loads Pictures library. I would like to show you the first picture url , But I could not do it exactly
How do i help please
I could not do it with that
A similar code:

functions.php
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)) {
    $first_img = "/path/to/default.png";
  }
  return $first_img;
}

loop.php
if ( get_the_post_thumbnail($post_id) != ” ) {

echo ‘‘;
the_post_thumbnail();
echo ‘‘;

} else {

echo ‘‘;
echo ‘<img src=”‘;
echo catch_that_image();
echo ‘” alt=”” />’;
echo ‘‘;

}

I send mail thank you