How to set Class for a particular image style?
In Drupal 8 you can do this with hook_preprocess_image in your themes mytheme.theme file.

/**
 * Implements hook_preprocess_image().
 */
function mytheme_preprocess_image(&amp;$variables) {
  if ($variables['style_name'] == 'my_custom_image_style') {
    $variables['attributes']['class'][] = 'my_css_class';
  }
}