Each page of the site isn't all that different from the rest. They could just load the page once and update the page content with a JS AJAX call and DOM manipulation, honestly. You wouldn't even need to send full URLs for the images that way, just the unique part of the URL such that JS could reconstruct it from a constant prefix.
We actually do this with reply loading already, but not by default. We have a read-only JSON API and use that to append new replies when you're browsing in a thread (click [Update] or [x Auto] at the top/bottom of a thread). We also have a de-pagination feature that grabs all of the OPs from a board and let's you scroll through the indexes as one giant page.
