I understand your argument, and it makes sense; however, I disagree. The user clicks the image in order to see it full-size. The lightbox overlays the page tha

I understand your argument, and it makes sense; however, I disagree. The user clicks the image in order to see it full-size. The lightbox overlays the page that the user is on. The user is not conscious that they are navigating to a new page, or even just a part of the page that they weren't at before (to the #image). Though the workflow is in actuality following a link, the user is not aware that they are following a link, so the back button functionality doesn't work as expected. Links are well understood by everybody that uses the Web. They change the page that you're on to a new page, or a new part of the page. Lightboxes were invented so that the user doesn't need to leave the page, or the part of the page, that they are on. So in my mind, lightboxes that don't behave like lightboxes and instead behave like links are broken.