The excerpt from "The Adventure of the Mystery Picture" that conveys the playful tone of the passage is:
"A picture!" said the old gentleman with the haunted head. "A picture!" cried the narrator with the waggish nose. "A picture! a picture!" echoed several voices."
In literature, playful tone refers to a tone spoken by the character which is fun, in high spirits, intended to be funny, or to make people laugh. This is usually done in a part of the story that it is not really important to the flow of the plot, so it doesn't distract the reader.
