After looking for a simple lightbox solution, I was left surprised that almost all of them use jQuery, which I didn't want to use in my projects anymore (unless necessary).
Litebox is the result of that work. It's an image-only lightbox with customizaton options (selectors, labels, controls, animations, etc.) and weighs around 12 KB. Currently, version 0.8.3 it's still in beta development but can be used on websites today.