1The only known image of Albert Einstein working on his theory of relativity
2The only known photo of Billy the Kid with the Lincoln County Regulators
The 4-by-5-inch tintype shows the Kid, and several members of the Lincoln County Regulators, playing croquet with friends, family, and significant others in the summer of 1878, according to Kagin's Inc., a numismatic firm that specializes in U.S. gold coins and collectibles.
Jeff Aiello, executive director of the documentary entitled Billy the Kid - New Evidence says, "This is the first photograph ever discovered of Billy the Kid with The Regulators, and that's significant in American history." (Source)
3The only known photo of one of 272 Georgetown University slaves who were sold to the state of Louisiana
The photo above is the only known photo of a man named Frank Campbell, who was one of the 272. Campbell was enslaved on a Jesuit plantation in Maryland. In 1838, he was shipped to a sugar plantation in Louisiana along with many others who were sold to raise money for the university. (Source)
4The only known photo of the musician known as the "Father of the Delta Blues"
Patton would sometimes hold the guitar on his lap and play it like a zither, as seen here. His wardrobe caught the eye of historian David Evans—Patton's collar and lapel are curiously hiked up on his left side, which may have hidden the scar on his neck from a near-fatal razor attack earlier that year. (Source)
5The only known photo of George & Willie Muse, who were kidnapped and forced into the circus, with their captor
In 1927, while visiting their hometown, their mother finally tracked them down and threatened to sue the circus. The Muse brothers were freed before returning to show business on their own terms a year later. (Source)
6The only known image of Marilyn Monroe, President Kennedy and brother Bobby Kennedy together
The black-and-white photo, taken by White House photographer Cecil Stoughton, showed Monroe still wearing the infamously tight-fitting dress she wore when singing earlier at Madison Square Garden.
Filmmaker Keya Morgan owns the original and says, "There is no other known photo of Bobby [Kennedy] with Marilyn or JFK with Marilyn, and it's not because they were never photographed together. In fact, they were photographed together many times, but the Secret Service and the FBI confiscated every single photograph."
Stoughton, who sold the image to Morgan a year before his death in 2008, told him agent missed one negative in their search. "The Secret Service came in when he was developing the negatives and confiscated all the ones of Jack, Bobby with Marilyn. The only one that survived is the one that was in the dryer." (Source)
7The only known image of El Santo unmasked
8The only known image of Jocelyn Gordon Whitehead—the man who killed Houdini
Whitehead's blows to Houdini's abdomen either started, contributed, or covered-up the appendicitis that would take Houdini's life nine days later on Halloween.
Whitehead, who is holding a book, had a strange history with them—he gained access to Houdini's dressing room with the claim of returning a borrowed book and was also charged with shoplifting books twice in 1928. He died a recluse and hoarder in 1954. (Source)
9The only known photo of the real military officer who was the inspiration for the lead character in The Legend of Sleepy Hollow
10The only known photo of the first African American man elected to the U.S. Congress