custom template tags?

This site is using ACF custom fields on images for things like Location, Photo credit, and Copyright. Note that the images have captions, but the captions do not render in the lightbox. If I copy the caption to the description field (which does not always appear in the back-end interface), then it renders as item.description.

I would *really like* for there to be a template tag “item.caption” (I’ve seen such a tag referenced in other documentation, but it doesn’t work). I’d also like to be able to incorporate ACF data into the lightbox.

Is there a way to do this? You reference add-ons, but I can’t find docs for writing add-ons.

This seems like it *should* be something that’s easy for a developer to do, and I seriously doubt that I’m the only dev that’s trying to do this. But developer instructions and how-to’s are either non-existent or impossible to find.

The page I need help with: [log in to see the link]