There must be some kind of glitch as i have just double clicked every image on this page(which takes me to the image on postimage) and the "Download Original Image" in the top left corner was there.
I have just repeated double clicking each image and at first i just get the image with black borders (no text at all), but if i refresh the page it all goes back to normal showing the download button.
I have seen this a few times before when postimage servers are running with a high load, it always goes back to normal, just Hang In There.