The drawings I have seen are very general as mentioned, simply expressing facets of what is described in Yasen Kanna. I would not say they are crucial to understanding the practice of naikan as transmitted in our line, and one could not learn the practice from such drawings without receiving the oral instruction. They are simply illustrative accompaniment to what Hakuin said and so useful in that regard.
Yes of course it was only a picture, and one has to have oral instruction. What struck me really was, that the whole thing was in colors. Very unusual. Another thing is that somehow I trace it to Kenninji, it was there where I have seen it, but only good photo of it, not the original. So it could be one of their treasures. But they do not show it to anyone basically. So one cannot find it in any picture catalogs of this monastery.
I had very short explanation of the three basic methods, since I did not intend to practice it at that time I did not ask further. I also got some explanation how it works, so one has to go through the test of diagnosis, like pulse etc. to decide which method should be used. I was only a listener of a very kind talk... I used myself the naikan method ages before the meeting, maybe for about three years and it was very helpful, and the talk I heard happened about 10 or 12 years ago. So not much is left in my memory. It is interesting that it has also some similarity with Tibetan practice of Vajrasattva... there is one very famous and beautiful big figure of Vajrasattva in Toji, Kyoto, but I do not know if Shingon is using in the same way like Tibetan schools.
However I never heard or read in Tibetan context so meticulous explanation of the nectar like the one by Hakuin. It is sort of crucial also for the success in this practice. Anyway Hakuin pointed that it gives all possible qualities of attaining enlightenment.