This extension picks up all the images from the given post and uses the fancy box to display a slider gallery with small thumbs at the bottom.
I don't understand why it behaves like this. It's a fancybox extension, not a slider gallery extension.
There's another extension that works correctly: https://discuss.flarum.org/d/19535-fancybox-extension-beta
It isn't updated in a really long time. I hope this extension gets updated to only display the clicked image, instead of showing a gallery of all the images.