What exactly is this VLA radio image of Jupiter showing?
A uniform disk has been subtracted to better show the fine banded structure on the planet.


This means the surface of the planet is far brighter (in the raw data) than the surrounding magnetosphere. The planet itself has been darkened considerably in the image, otherwise it would be a pale circle and show little or no detail.

The press release from the VLA's site itself doesn't even show the wide field image however; it concentrates on the image showing detail of the spot itself. It mentions:


  an innovative data-reduction technique to "unsmear" the data from many hours of observing


There is an earlier 2006 paper by some of the same researchers showing earlier attempts, with a much more coarse radio map of Jupiter. This goes into more detail about the earlier image processing techniques though (requires good knowledge of Fourier analysis etc.)

(Note: I can't read the current Science paper in full, it's behind a paywall.)