Rosalind Franklin / Raymond Gosling, 1952. X-ray diffraction image of DNA from which Watson + Crick derived the double helix.
From Wikipedia
Photo 51 is a 1952 X-ray based fiber diffraction image of a paracrystalline gel composed of DNA fiber taken by Rosalind Franklin's PhD student Raymond Gosling at King's College London, while working in Sir John Randall's group; Maurice Wilkins later shared the image with James Watson without their permission. The image was tagged "photo 51" because it was the 51st diffraction photograph that Gosling had taken. It was critical evidence in identifying the structure of DNA.