add function to child theme function.php

Hi I am using a child theme, I am a total beginner, I am not very good at understanding the code and would like to keep things clean and tidy.

so i did some copy & paste as told and got it work finally:

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;
}

Yet I would like to ask if there are any ways I can add the function catch_that_image() code in a child theme function.php, instead of putting it directly into the original parent theme?

i tried to do paste it inside a function.php in child theme any it only get errors. anyone would help me with some example?
thx~

http://wordpress.org/extend/plugins/get-the-image/