Can I get the ID of an inserted / linked image?

Question

I am using this code to get the url of first inserted image in a post. Is there any way I can get the ID of that image.

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