Here is a rare picture of Mascagni at the piano which I recently acquired. The handwritten text on the back of the picture says that it was taken in Rome by "Ettore Patrini, editor & [...]" (last name and function are hard to read). A number on the back could read "17", for 1917. To be broad, I think that the picture was taken between 1900 and 1920, but I have trouble pinpointing an exact year.
On the full-size picture, you can see that Mascagni is smoking a little cigar, as is typical. The quality of the picture doesn't allow seeing what is on the score.
For more photos of Mascagni, see Mascagni at the Piano.
Update: Roger pointed out to me that this is the same piano and location as in this photo. If the information is correct and this was in fact taken during Isabeau's composition, or even at Illica's villa in Castell'Arquato, then it is likely that the two photos were taken not far apart. The composition of Isabeau started in late 1908, and the opera premiered in 1911, which gives us a better range. If it was in Castell'Arquato, then it was likely during June 1910.