Photos made after 4/2006 will most likely be Nikon D200, up to 10 megapixel images. Prior to that photos had been made using a Nikon 8700, 8 megapixel camera. There's a bit more to it than that; the D200 has a much larger sensor (each pixel is considerably larger, hence clearer) and the lens is much better.
Photos by Michael Gordon. The more recent ones use NEF (a proprietary "raw" file format) as the original file format.