Image submit buttons on Drupal 5.10

I follow this useful howto by Tom Constant on how to add image submit buttons on Drupal forms. Since I got a JavaScript error on drupal.js (line 31: button is undefined), I rewrite Tom’s implementation, and now it works fine on Drupal 5.10:

function phptemplate_button($element) {
  // Make sure not to overwrite classes.
  if (isset($element['#attributes']['class'])) {
    $element['#attributes']['class'] = 'form-'. $element['#button_type'] .' '.
 $element['#attributes']['class'];
  }
  else {
    $element['#attributes']['class'] = 'form-'. $element['#button_type'];
  }

  // here the novelty begins: check if #button_type is normal submit
  // button or image button
  switch($element['#button_type']) {
	case 'image': $button_type = 'image';
	break;
	default: $button_type = 'submit';
	break;
  }
  return '<input id="'. $element['#id'].'" name="'. $element['#name'] .'"
 type="' . $button_type . '" value="'. check_plain($element['#value']) .'" />