Hvorfor vises mit billede ikke?

Du indstiller ikke den korrekte Content-type header, før billeddataene ekkoes.

Du SKAL også undslippe $_GET['id'] parameter.

// Escape $id
$id = mysql_real_escape_string($_GET['id']);   

$link = mysql_connect($host, $user, $passwd);
mysql_select_db($dbName);

// Use the escaped $id
$query = "SELECT picture FROM products WHERE serial='$id'";
$result = mysql_query($query,$link);

if ($result) {
  $row = mysql_fetch_assoc($result);

  // Set the Content-type
  // This assumes image/jpeg. If you have different image types,
  // you'll need logic to supply the correct MIME type
  // image/jpeg image/png image/gif, etc
  header("Content-type: image/jpeg");
  echo $row['picture'];
}
?>

I dit hovedscript ser det ud til, at du blot mangler et echo

        <td><?php '<img src="getImage.php?id=' . $row['serial'] .'"/>'
        // Should be
        <td><?php echo '<img src="getImage.php?id=' . $row['serial'] .'"/>'
        // ------^^^^^^