Picture of the king of Bohemia and his mistress
The King of Bohemia was an official mentioned in a Sherlock Holmes holodeck program played by Data in 2365.
While acting in the role of Holmes, Data informed Inspector Lestrade that a photograph of the king and his mistress was about to be used for blackmail. (TNG: "Elementary, Dear Data")
The character, called Wilhelm Gottsreich Sigismond von Ormstein in the Holmes stories, was introduced in the story A Scandal in Bohemia and additionally mentioned in others. Notably, the king is mentioned to have presented Holmes with a snuff box for his help in the story A Case of Identity. These two stories are both referred to in the episode, but are out of order: initially Data showcases a snuff box implied to be a present from von Ormstein for solving his case, only for Data to be presented that case shortly later. Since the first mention only calls the character von Ormstein, and the second only refers to the king of Bohemia, the two might actually be considered different characters within the world of Star Trek.
During the time period of the story, Bohemia had long ceased to have its own royal family, the function instead being filled by the Austrian Emperor.