The Lightbox Popup feature of ACF Gallery allows you to click on a thumbnail and display the full image in a lightbox popup. This feature is enabled by default and is based on Glightbox, a lightweight and dependency-free (no jQuery) lightbox script that is only 11kb gzipped. Additionally, you can choose a module to be displayed below the image’s caption in the lightbox popup and enable the optimization of the original image uploaded by the user to increase loading speed and free up storage space.