Having met Justin Quinnell last year, he explained to me how he did this, the images were actually burnt into photographic paper that he used, which was rated at about ISO 3. he never developed the images as such, but just skips straight to fixing them and then scans them into photoshop. when you have an exposure like this, it will burn into the paper at its own accord. the pinhole in the camera he used was made using the head of a small pin, and had a rough aperture of f/90