code: public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
      $image_styles = image_style_options(FALSE);
      $form['landscape'] = [
        '#type' => 'select',
        '#title' => t('Landscape image style'),
        '#options' => $image_styles,
        '#default_value' => $this->configuration['landscape'],
        '#description' => t('Select the image style for landscape images'),
        '#required' => TRUE,
      ];
      $form['portrait'] = [
        '#type' => 'select',
        '#title' => t('Portrait'),
        '#options' => $image_styles,
        '#default_value' => $this->configuration['portrait'],
        '#description' => t('Select the image style for portrait images'),
        '#required' => TRUE,
      ];
      return $form;
    }
docs: {@inheritdoc}