There's an interesting theory proposed by Holy Grail writers Lynne Picknett and Clive Prince: the first photograph ever taken is actually Leonardo's great hoax The Shroud of Turin. The book is called "The Shroud of Turin: In Whose Image?"
In the context of the Shroud's undisputed photograpic qualities it is a sound theory, and the authors have successfully demonstrated a methodolgy for reproducing Leonardo's Shroud using materials available to the Master in his day.
They even go so far as to assert that the man on the Shroud is Leonardo himself! It's not so far-fetched if you compare the Shroud Man to any of Leonardo's self portraits, and if you think about it - no-one else in that era had the wit, the resources or the blasphemous sense of humour to pull off such a stunt.
I for one do believe the Shroud is a photograph. There's simply no other way to explain its blatant photographic negative quality. Artists at the time were familiar with the camera obscura, and some people *were* experimenting with the interaction of light and chemicals - Leonardo being the most famous and most likely to succeed. Interestingly, the authors claim the Shroud image is actually a burn mark, with the density of burning corresponding to the density of chemicals left after "fixing" the image subsequent to exposure. This was the hoaxer's work-around for getting a permanent image with inadequate chemicals available at the time.
I forget the name of the documentarian but I remember seeing a recent program (BBC I believe) about an art historian who believes he can demonstrate the use of lenses and camera obscura technology in the paintings of Dutch and Flemish masters many decades and in some cases centuries prior to the "official" invention of the device. He even goes so far as to show how the artist has unwittingly included lens flaws and chromatic abberation into their paintings! It certainly goes a long way to explaining the sudden appearance of photo-realistic paintings from a tradition of cartoon-like medieval art.
Food for thought!
Check out the Shroud of Turin and tell me it's not a photograph!