WordPress: Catch first image from post

catch-image.php

/*--- Catch the first image function use <?php echo catch_that_image() ?> ---*/

 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;
 }