How to add css classes to the <img> tag generated by an image field (Drupal 8)

theme_name.theme.php

<?php

/**
 * @file
 * Functions to support theming in the THEME_NAME theme.
 */

/**
 * Implements hook_preprocess_theme().
 */
function THEME_NAME_preprocess_field(&$variables, $hook) {
  $element = &$variables['element'];

  if ($element['#field_type'] === 'image') {
    $delta = 0;

    while (!empty($element[$delta])) {
      // Initialize the classes array in case it doesn't exist.
      $variables['items'][$delta]['content']['#item_attributes']['class'] = $variables['items'][$delta]['content']['#item_attributes']['class'] ?? [];
      $variables['items'][$delta]['content']['#item_attributes']['class'] = array_merge(
        $variables['items'][$delta]['content']['#item_attributes']['class'],
        _get_css_classes_for_image_field($element['#field_name'])
      );

      $delta++;
    }
  }
}

/**
 * Gets css classes for a given (image) field name.
 *
 * @param string $field_name
 *   The name of the field to have css classes added to.
 *
 * @return string[]
 *   An array of classes to be applied
 *   or an empty array in case none are available.
 */
function _get_css_classes_for_image_field(string $field_name): array {
  $classes = _get_image_field_class_mapping();

  return $classes[$field_name] ?? [];
}

/**
 * Gets a list of desidered classes by field name.
 *
 * This method in intended to use as the data source for adding specific
 * css classes to the <img> tag of some image fields.
 *
 * @return array
 *   The class mapping in the format {field_name: [desire_class_names]}
 */
function _get_image_field_class_mapping(): array {
  return [
    'FIELD_NAME' => ['CSS_CLASS_1', 'CSS_CLASS_2'],
  ];
}