/**
 * Implements hook_filter_secure_image_alter().
 */
function {{ machine_name }}_filter_secure_image_alter(&$image) {
  // Turn an invalid image into an error indicator.
  $image->setAttribute('src', base_path() . 'core/misc/icons/e32700/error.svg');
  $image->setAttribute('alt', t('Image removed.'));
  $image->setAttribute('title', t('This image has been removed. For security reasons, only images from the local domain are allowed.'));

  // Add a CSS class to aid in styling.
  $class = ($image->getAttribute('class') ? trim($image->getAttribute('class')) . ' ' : '');
  $class .= 'filter-image-invalid';
  $image->setAttribute('class', $class);
}