Holmes is being consulted by the King of Bohemia, who is being blackmailed by a noted opera star with whom he had had an affair. She has a photograph of the King and herself, and is threatening to send it to his fiancée, a Scandinavian princess. Holmes asks about the photograph:
"One other question," said he. "Was the photograph a cabinet?"
What does "cabinet" mean in this connection?

In the denouement, it turns out that a "cabinet" is a rather
large photograph intended to be kept on display on a cabinet, and so not easily hidden.