I think Google (and Ghost Forum) search is your best friend and you could start there. I did a 10 seconds research for you and these are the top results I got back (I’m not a developer, so I hope they solve your issue):
- https://www.ghostforbeginners.com/add-an-image-lightbox-to-ghost/
- https://forum.ghost.org/t/do-we-have-any-lightbox-for-ghost-gallery-card/4129
- https://stackoverflow.com/questions/25068803/add-lightbox-effect-to-images-automatically
- https://www.wdiaz.org/how-to-integrate-fancybox-3-on-ghost/
Cheers! 