This is a basic square gallery, using CSS, and a div that dynamically stays square, and sets the image as the background image, cover image. These are wrapped in the anchor tag to active the lightbox. This way you can have either a different image for the thumbnail, or a compressed thumnail.

I put the hvr-shrink effect on to show hover (I recommend that, hvr-grow, highlight, or your own manual hover effect), but it functions fine without that class.