Consider if this was done just HTML 4 and no CSS or Javascript. You would click a thumbnail and a new page would be loaded with a full size version of the image. You would then click a "close" button that would take you to the page with the thumbnails again. If you hit "back" you would go back to the image.
The way the page currently works follows the "stack" of what I actually visited by clicking links. If I want to reverse my workflow after visiting all 5 images and closing between them, I personally would expect the back button to go through all 5 images. From this perspective, doing the same effect with Javascript that doesn't update the stack might surprise me if after clicking 10 links on this page, a single click of the browser back button skips 10 steps of my workflow and returns me to HN.