NHK online reported July 3, 2006, that,
"The aerial was taken by a US Reconaisance plane April 6th, 1945,off Tokuyama in Yamaguchi prefecture, western Japan, five hours before the Yamato made its final sortie.
The Yamato Museum in Kure City, Hiroshima Prefecture, recently obtained a digital image of this photo, which is stored in the US National Archives in Washington. "
I would try contacting the National Archives, and see if you can also purchase a copy of the image. It is reported that, "the image shows the Yamato preparing for departure, and six other escort vessels, including the light cruiser Yahagi, which were anchored around the Yamato." And that it clearly shows anti aircraft guns installed near the stern.