Much higher-resolution versions are available on the HST site.
The astronomers took a series of images of Ganymede as it orbited Jupiter and made an 18 second/540 frame animation of it. Very cool! That's the advantage of observing bright objects: short exposure times means you can take lots of images, and in this case there is motion, too, so a movie becomes totally cool.
Credit: NASA, ESA, E. Karkoschka (University of Arizona), and G. Bacon (STScI)