I'm no astronomer, but I'll wager they get the camera's iris as open as possible, and then do digital enhancements (hence the light fuzziness in blacker portions of the picture). Since spacecraft sent out there are usually in a lot of motion, exposure times couldn't be that long. However, there is the possibility of moving into geostationary orbits around bodies, but I imagine that would require more adjustments and efforts than they would be willing to make if they wanted to fly by several celestial objects. At best, it's probably that if they pass by a moon or whatnot, they do some trajectory-work and as the craft passes by, it temporarily matches the orbit and revolution of the body it photographs, allowing longer exposure times.
...But I'm just guessing.
