The option is not available when using an image formatter like photoswipe or colorbox. I guess the module should support the most common ones?
List of image formatters that should possibly be supported:
https://www.drupal.org/project/image_url_formatter →
https://www.drupal.org/project/photoswipe →
https://www.drupal.org/project/colorbox →
https://www.drupal.org/project/image_link_formatter →
https://www.drupal.org/project/juicebox →
https://www.drupal.org/project/fancybox →
https://www.drupal.org/project/bg_image_formatter →
https://www.drupal.org/project/image_delta_formatter →
See https://www.drupal.org/project/project_module?f%5B0%5D=&f%5B1%5D=&f%5B2%... →
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