Hi,
There is only a counter on an HTML page if you're using the pageable layout. Images in the lightbox are not counted. Access to the image file could be in your server log, but if you don't use local images, this will not help (even then it is a pain).
I recently added a download log. This shows who used the download-button on your image, but this is actually not what you're looking for when it comes to the question of the "most viewed" images. Still, might be worth a look. Where is a very long article in the manual ;) https://www.svenbluege.de/joomla-event-gallery/event-gallery-manual/download-log