And while we’re on the subject, I would feel rather patronized—like a dog commanded to perform a trick—if someone presented me with a painting and said, “Say something mathematical!”
Well, the answer to that might be, “Give me some time, and I might find something interesting to say about it.”
What can mathematics say about paintings? The ancient Greeks probably thought a lot about things like that; you could talk about proportion and perspective and things like that, I guess. You could perform more a detailed analysis if you digitize the image and break it up into elements that aren’t obvious from a purely visual inspection, such as overall contrast levels.
I do like g’s suggestion, though.