This is a widely-known postcard created by John Towns, who, along with his wife, Dorothy "Dot" Towns, ran The Red House Bed and Breakfast.  It shows Beal and Bunker's original Mail Boat, Island Queen, now owned by Charles Liebow.

The caption on the back reads:
Mail boat "Island Queen" Arriving at Great Cranberry Island.
The shores of Mt. Desert Island in the background.
Original Photo by J.G. Towns, Holbrook, Mass.

photo courtesy of Dorothy Towns