CICLOPS has released a processed (and not jaggy) version of the Janus image taken last month showing dark spots on the surface. As mentioned at the time, Janus' surface is covered in darkish spots. Consider our experience with Phoebe. On that moon, the surface was quite dark, with bright spots seen on the surface at low resolution. At higher resolution, those spots were revealed to be small impact craters which punched through the dark surface layer to bright icy interior. The impact had brought up material from that bright interior and had deposited it on the dark surface. A similar effect maybe happening on Janus. The dark spots could indicate that the interior of Janus is darker than the surface and small impacts bring that dark material to the surface. The individual craters are not resolved in these spots, except for a comparatively large impact at upper right. That crater has dark material surrounding it, perhaps conforming to this hypothesis. There are other craters in this view, similar in size to the one at upper right, that don't have dark material surrounding them. Somehow, a space weathering process is brightening the surface, perhaps from ring material.
The original image has a resolution of 2 km/pixel though this view has been magnified 2x to enhance feature visibility. This image was taken on May 20, 2005 from a distance of 357,000 km. The phase angle on this image is very small, only 6 degrees so most of what you are seeing here are albedo variations, not topographic shading.