Lightbox gallery

I cant seem to be able to add another lightbox image inside the already made lightbox gallery layout feature on my site. Ive tried adding another lightbox link in the grid container, but it doesnt work, if I copy the other boxes i cant change the picture. How do I add another “box” that looks the same as the other, in the remaining grid containers?

Here’s a read-only link to my site. (the gallery is at the bottom of the page)