There's a great thread going over on Quora discussing some of the most important in science. Among them is an iconic image of James Watson and Francis Crick, who in 1953 suggested the first correct double-helix model of DNA. Here's another image of the duo with Maclyn McCarty. And, if you'd like to dig deeper, there's an essay in the journal Isis discussing the history of the image on Quora.
Photo courtesy Public Library of Science