To open multiple images in a lightbox when clicking a region or marker, you’ll need to manually add the multiple links to the action content of the region or marker.
If you’re using an image from your website, you can get the full URL going to Media > Library, select the image you want and click ‘Copy Link’. That link can be used in the ‘Action Content’ field.
Once you have the full image URL you just need to place it in the ‘Action Content’ field. You’ll repeat this process for as many images you need.
To open multiple images in the lightbox, as a slideshow/gallery, you can add multiple links to images divided by commas:
https://yoursite.com/wp-content/uploads/2021/08/porto.jpeg, https://yoursite.com/wp-content/uploads/2021/08/lisboa.jpeg, https://yoursite.com/wp-content/uploads/2021/07/coimbra.png
In the settings page of the plugin, in the editing tab, you can set the ‘Action Content’ field type to textarea, in case you’re seeing a small text input.