If your theme uses the Lity lightbox, you should enable this. By doing so, the plugin will check for the Lity lightbox component in your pages, and it will replace its images with properly resized and optimized ones.
If you are not sure whether you are actually using Lity, follow these instructions:
- Go to any page that has images opening on a lightbox (What is a lightbox?)
- Press Ctrl + U (⌘ + Option + U on macOS) to display the source-code
- Press Ctrl + F (⌘ + F on macOS) to open the Find Bar
- Look for the text
data-lity - If you get any results, then you are using Lity and you should enable this option.