All the images in that collection are 1024 pixels on the long side at original size. What's actually shown to you by default on each images' own page or in the slideshow are all 640 pixels on the long side. I don't see anyway to get around this as they give you no option to view the slideshow in original image size.
My monitor is a 42" LCD with a 1920x1080 resolution. That's not the issue though.
 Regarding your example image, I had already tried that option in the past thinking it might work. All it does is scale the smaller 640 pixel image to fill the screen. It doesn't show you the original size image with no scaling.--
Photos By Greg Strong