This image is a slightly cropped 2 x 2 mosaic comprising images taken on 13 December 2014 from a distance of about 20 km to the centre of the comet, or 18 km from the surface. At this distance, the resolution of Rosetta’s narrow-angle camera is about 34 cm per pixel, and the full mosaic covers roughly 1.3 km.
More information: The quest to find Philae