It's clearly nonsense anyway. Even if you accept the argument that he has no claim to the copyright of the original image as it existed in his camera (which I'm not sure I do), the image that he actually released was almost certainly a derivative work that he had made. Assuming that that derivative work included any significant modifications, e.g. framing, color, etc I would expect it to be copyrightable and that copyright would clearly belong to Mr Slater.