The websites of BBC News, the German magazine Der Spiegel and the television network France 2 (among other outlets) report the discovery today. The surviving print is not the original daguerreotype but a copy dating from the second half of the 19th century.
The photo was taken in Alt‹tting in 1840 when the 78-year-old Constanze was visiting her friend, the composer Max Keller. She can be seen in front at the far left, sitting next to Keller. It is evidently the only photo ever taken of her.
The image is one of the oldest surviving examples of photography in Germany. The daguerrotype was perfected and patented in France in 1839.
Mozart died in 1791 at the age of 36; Constanze was 29 at the time. She later married a Danish diplomat and lived until 1842.