One by one I'm afraid, but I would like to discourage you from doing that. Believe it or not, but bulk of people are still on slower internet connections (under 1.5Mbps) which means that post would take days to load, even with highly optimized images. This just isn't a good idea. I would never wait for that post to load, I would just go elsewhere.
Secondly, the search engines consider page loading times and total page download size when calculating search engine ranking. That page, quite simply, would be completely missing from all search engines, or if they did list it, it would be down on page 1,976,540. Effectively buried and ignored.
300 images, even if they are only 20k each, is over 6MB just in images.