Jupiter and the Galilean Moons
Jupiter's Great Red Spot with, from top to bottom: Io, Europa, Ganymede and Callisto
The four Galilean moons, individually.
Left to right: Io, Europa, Ganymede and Callisto, showing comparative size.
Io and the Earth's Moon together. Io is slightly larger. This picture is taken from Solarviews.
Images taken by the Galileo and Cassini probes, and are in the public domain. You can find more superb shots of all things astronomical at the Astronomy Picture of the Day website.