When it comes to the gallery-n method. The idea is simple:
1) if you give all your images displayed in a post or page just a
rel="lightbox" attribute, it will be displayed together in one gallery
2) if you assign
rel="lightbox[gallery-1]" to 10 images, and
rel="lightbox[gallery-2]" to 5 images, you will have 2 galleries on a page containing 10 or 5 images
3) if you give every image in a post a differently named rel attribute for e.g.
rel="lightbox[image-3]" etc. each image will be displayed separately, not as a gallery (so you can't navigate between images)
Basically whenever there are same rel attribute names assigned to images (or videos) those files will be a separate lightbox gallery.
But from the screenshot you've sent I think you want to achieve something different: to make one image (thumbnail) open a gallery of images, that are not present on the page/post. Do I understand you correctly?