Long exposure and high sensitivity on the sensor.
This.
If you know you're going to be going 3 billion miles away from the Sun, you should pack accordingly.
I wonder what the exposure times really were though? As fast as that thing was going, I can't imagine that they wanted to spend a lot of time holding in one place to get an image or measurement. Long exposure would also potentially mean blurring from the shifting parallax.
Edit: Looks like I'm not the only one to wonder.
100millisecond exposure time.
The Voyager probes did some long exposures, but they also didn't have cameras quite like we send out these days.
The probe has a really bright flash.
Really bright: "Mysteriously abundant oceans discovered on Pluto!"