Often called "Einstein's Tongue", is an image that became iconic due to its humorous character. Humor as itself requires intelligence, and capturing humor in photography (especially when done candidly) is one of the hardest things to do in the discipline. The picture shows a different profile of Albert Einstein, and it's playful and nutty character is what makes the image so great.
The moment occurred in the 72th Albert Einstein's birthday celebration, and a lot of photographers were there, but just Sasse capture the one that became iconic. We can delight ourselves with a humorous Einstein, instead of the Nobel prize-winning physicist who developed the theory of general relativity, therefore, we have a more accessible side of Albert Einstein.
A really important background fact of the image, is that Einstein enjoyed it so much that asked the UPI (United Press International) to facilitate him nine copies of the cropped image for his personal use. One of those personal uses landed on Howard K. Smith, a friend of Einstein. The image, as predictable, had a little text at the back, the text quoted this "This gesture you will like, because it is aimed at all of humanity. A civilian can afford to do what no diplomat would dare."
Source-
https://www.phototraces.com/creative-photography/world-famous-photos/