This fascinating photo recently turned up at an eBay auction. It was advertised as a photo of Mascagni rehearsing Cavalleria rusticana, but obviously the sets are those of Aida, and if you look carefully you can see the conductor score for Aida right behind Mascagni!
On the podium, you can read "Fot. Willinger Wien". This inscription is designed to look like something engraved on the podium, but it was likely added by the photographer. This would point to a performance of Aida in Vienna. As for the date, I would guess that this took place during the 1930s given Mascagni's looks. It should be possible to find a more precise date by looking up when Mascagni conducted Aida in Vienna.
Thanks to the seller, James Camner, who was kind enough to provide me with a good scan of this photo.
Update: Roger points out that this looks like an outdoor performance, and that Mascagni conducted Aida in Vienna in August 1924, among other dates. So that photo could very well be from August 1924.
Update: John found this mini-biography about a photographer called Laszlo Willinger who had a studio in Vienna in the 1930s.