Image Description is not available for views

Thanks for the report. Looks like the hook_views_data can be updated in photos.module. Around line 1709. Using the same settings as title should be fine:

  // Image description.
  $data['photos_image']['des'] = [
    'title' => t('Image description'),
    'help' => t('The description for this image.'),
    'field' => [
      'id' => 'standard',
    ],
    'sort' => [
      'id' => 'standard',
    ],
    'filter' => [
      'id' => 'string',
    ],
  ];