In reply to Morgan Woods:
Dunno what it is called, but I do know how it is done.
It's quite easy to do if your camera has burst mode and you have a tripod. If you don't have a tripod then something like the align_image_stack program that comes with hugin might be enough to line the images up and let you do it hand held.
Simply load each image as a layer in your favourite photo editor and selectively erase/merge down to uncover the dogs.