As I’ve mentioned in my post, I only have a very empty and basic WP version on my local computer, so there’s no link to share. There are no other plugins installed other than supplied with WordPress itself. All I did after installing WP, was add a linked picture to the example post and install lightbox plugins, removing them once they didn’t appear to be working.
Luckily, I managed to get things working with the Responsive Lightbox plugin. As long as I don’t check the ‘conditional loading’ option, it works! Yay!