This is on a local site but you can easily reconstuct it.
I am creating a image gallery like you did in the demo movie with the same settings you used. I have 21 images and I set the POSTS PER PAGE to 8, PAGINATION, AJAX PAGINATION. Images are set IMAGE LIGHTBOX, IMAGE LIGHTBOX GALLERY.
When viewing the page I see the first 8 images, I click LOAD MORE once and then again to see all 21 images.
I click on any image of the first 8 and they open in the lightbox and I can go through the images in the lightbox.
PROBLEM: After 8 images (or what ever number you set in POSTS/POST PER PAGE) it loops back to the first image and does not show the remaining images.
Now if you click on any other image after the first 8 images i.e., 9-21 the images do not load in the lightbox but on their own page.
So in plain speak, what ever number you set in POSTS/POST PER PAGE will appear in the lightbox. The rest are not loaded in the lightbox. Even though all images are loaded on the page with the Ajax Pagination.