Dogs can categorize pictures into those that contain dogs and those that don't

From New Scientist

Four dogs were simultaneously shown photographs of a landscape and of a dog, and were rewarded if they selected the latter using a paw-operated computer touch-screen. When the computer-savvy dogs were shown unfamiliar landscape and dog photos they continued to identify those containing dogs. And when shown an unfamiliar dog superimposed on a landscape used in the training phase, they were still able to pick it out in preference to an image of just a landscape.

Is this surprising? It would be if one thinks of dogs as computers and marvels that they can do image recognition. It isn't if one thinks this asks whether dogs can recognize other dogs. It may be if one thinks this asks whether dogs can understand that they are being asked to select pictures that contain other dogs. Can dogs distinguish pictures of collies from pictures of Beagles? Can they distinguish pictures of other dogs from pictures of their masters? Can they distinguish pictures of squirrels from pictures of rabbits? The experiment, at least as described, didn't ask useful questions.