defaultImageForm example

$scheme_options = \Drupal::service('stream_wrapper_manager')->getNames(StreamWrapperInterface::WRITE_VISIBLE);
    $element['uri_scheme'] = [
      '#type' => 'radios',
      '#title' => $this->t('Upload destination'),
      '#options' => $scheme_options,
      '#default_value' => $settings['uri_scheme'],
      '#description' => $this->t('Select where the final files should be stored. Private file storage has significantly more overhead than public files, but allows restricted access to files within this field.'),
    ];

    // Add default_image element.     static::defaultImageForm($element, $settings);
    $element['default_image']['#description'] = $this->t('If no image is uploaded, this image will be shown on display.');

    return $element;
  }

  /** * {@inheritdoc} */
  public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
    // Get base form from FileItem.     $element = parent::fieldSettingsForm($form, $form_state);