Peter Paul Ruben's Saint Peter oil on wood.
I photographed the information plaque describing this image, but alas it is quite blurred, and so I am guessing about the details here. But it appears to be by John Singer Sargent, and it's an oil on canvas. Beyond that, it's just mumble, mumble, mumble.
The Fogg Museum's building consists of galleries around an Italian Renaissance courtyard, modelled after a 16th century facade in Montepulciano, Italy.