The relative brightnesses of the other moons is approximate. It appears to show surface markings. Filters can make a big difference on the appearance of Jupiter's clouds. Longer red wavelengths penetrate deeper into the Jovian atmosphere. The image on the right is among the best I've obtained.
Longer wavelengths are also less affected by Earth's atmospheric turbulence.
Use of a red filter can potentially yield higher resolution images. The right image is a high resolution simulated view of Jupiter made by the Casinni spacecraft on December 7, 2000. About wo weeks separate the two images, although many of the same features, including interesting cloud features near the Great Red Spot, can be identified in both images. The shadow of Io is over the southern equatorial belt. The larger shadow of Ganymede grazes the southern polar region.