The key X-ray diffraction image used by Watson and Crick was Photo 51. It was a striking diffraction photograph of DNA in its hydrated B-form and provided powerful evidence that DNA had a helical structure.
Photo 51 was produced at King's College London in 1952 by Raymond Gosling, a doctoral student working under Rosalind Franklin. The image was the 51st diffraction photograph in Gosling's series, which explains its name. Its distinctive X-shaped pattern is characteristic of a helix; the spacing of the reflections also helped indicate important dimensions of the DNA molecule.
Maurice Wilkins showed the image to James Watson in early 1953. Watson and Francis Crick combined clues from Photo 51 with other experimental results, chemical data, and model-building to propose the double-helix structure. Their landmark paper appeared in Nature in April 1953.
A common mix-up is to call Photo 51 a photograph taken by Franklin herself. Franklin supervised the work and was a major DNA researcher, but Raymond Gosling actually operated the experiment that produced the image. Her data and analysis were nevertheless central to understanding DNA's structure.