Is it possible to add template suggestions for image styles?
hook_theme_suggestions_HOOK_alter doesn't use a third argument.
You are supposed to add the suggestions as string.




/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function MYTHEME_theme_suggestions_image_style_alter(array &amp;$suggestions, array $variables) {
  $suggestions[] = $variables['theme_hook_original'] . '__' . $variables['style_name'];
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function MYTHEME_theme_suggestions_image_alter(array &amp;$suggestions, array $variables) {
  $suggestions[] = $variables['theme_hook_original'] . '__' . $variables['style_name'];
}