If your screenshot represents exactly what you actually did then it would seem firstly, that your paths are incorrect.
"path-to-fancybox-js-files" is not a real path, it's just an example.
Also, you need to set your images as links.
I managed to get this working in moodle 2.8 (2.9 uses JS as AMD and I don't know what would happen if you tried this there...).
See the attachment (based on Itamar's official explanation page you mention at the beginning of your post, but with a couple of "helper" additions)