Add Drupal 7 native image lazy loading by template hook

Modern browsers support native image lazy loading by loading="lazy" attribute!
See: https://caniuse.com/loading-lazy-attr

Since Drupal 9.1 this is also done by default in Drupal: https://www.drupal.org/node/3173719

To add this attribute to your Drupal 7 projects, you can use the following preprocess hook in your themes template.php (or module):

/**
 * Implements HOOK_preprocess_image.
 *
 * @param array $variables
 */
function THEMENAME_preprocess_image(&$variables) {
  // Add lazy loading attribute to all images:
  $variables['attributes']['loading'] = 'lazy';
}