Get First Image from a WordPress Post

get_first_image.php

get_thumbnail_image_src() {
  /**
  * @todo 
  *  * Return or echo
  *
  *
  */
  global $post;

  if ( has_post_thumbnail( $post->ID ) ) {
	  $src = wp_get_attachment_image_src( get_post_thumbnail_id() );
	  return $src;
  }

  $first_img = '';
  $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 = get_bloginfo('template_directory') . "/images/default.jpg";
  }

  return $first_img;
}

This function retrieves the source of the first image of a WordPress post. But if the post have a post thumbnail, it will return the source of it.