At a public lecture Einstein gave in Pittsburgh in 1934, four hundred students were present when Einstein mathematically derived his famous mass-energy equivalence equation: E=mc2. This is thought to be the only surviving photo that shows Einstein working on that derivation, pulled from a halftone newspaper clipping by David Topper and Dwight Vincent of the University of Winnipeg, who discovered it in 2007.
If you look closely, you’ll see the mass-energy equivalence in the lower left hand corner of the blackboard on the right. You might notice that the famous equation says ΔE0=Δm and E0=m instead of the expected E=mc2, Topper and Vincent explain in their paper: "It is the right blackboard that contains the equation. But its format may disappoint or confuse the average viewer, because from the start of the lecture Einstein employed the convention of setting the speed of light c to unity. Hence a close look at the lower left section of the right blackboard in [the picture] reveals the relation ΔE0=Δm, and below it is E0=m. As far as we know, [this photo] is the only extant picture with Einstein and his famous equation."
The daily Galaxy via http://www.pittsburghmagazine.com