Figured it out. I think this may be a bug with the theme. I’ll post my solution here for the sake of others with the same problem and for the dev to have a look.
When I create a single image in the Swift Page Builder, I did notice that upon adding an image, the thumbnail box was blank.
If you just add the image and preview the site, the images are not linked to anything ending in .jpg. Therefore, the lightbox is blank when you click it.
Simply going back into the Swift Page Builder, clicking edit on each image, doing nothing except clicking save (you’ll notice the thumbnail is there now), and update the whole page will fix everything. The images now link to .jpg files and the lightboxes load as expected.
