A series of such pictures over time would even be better. The idea is to learn more about the structure of the storm by observing how the shadow behaves as it is passing over the boundaries and body of the Great Red Spot storm.
Because of the current orientation of the plane of the satellites orbits, only Io's shadow falls on the Red Spot. Since Io is the fastest moving of Jupiter's major moons, it's shadow must fall on the Red Spot fairly often. I'd scan the various archives of Jupiter images to look for images that meet this criterion: