another possibility: the photo is a hoax, one of many hoaxes perpetrated by snake-oil salesmen and other such conmen in the late 19th and early 20th centuries - remember the Gypsum Giant?
Yes, you are 100% right. I don't doubt that the picture might be a hoax - you can't beleive everything you see, you know.
Yet that doesn't explain why many folks have a memory of it, but nobody can find the pic. A hoaxed picture shouldn't just vanish like that - a fake photo in a 1955 book could make it into a 1990 or 2000 book, even if it reveals the pic is a hoax.
The controversy isn't so much on the validity of the pic (the truth theirin), but more on 'why do so many people have a vivid memory of a photo that nobody can seem to find?'