I think they restrict photgraphy, then sell books with the photos. Similar if not just like Bill Valvanis did with the US National exhibit. Have to admit I am not posative about this. But for the time I have been on line I haven't seen but an occasional photo of these trees, most likely they were after event photos.
you can try looking through the Kindai Shuppan site: http://www.bonsai.co.jp/
and the Nippon Bonsai Association site: http://www.bonsai-kyokai.or.jp/
There are gallery pages of some of the top bonsai in Japan, many of which were in Kokufu ten at one time or another. The pictures may not be from the Kokufu ten albums but many were exhibited there nonetheless.