Scan your eyes quickly across the full size image a couple of times. The more you wiggle your eyes, the more it moves.

If I just fix my eyes at one point in it, it does not move for me either. I suppose people for whom it wriggles all the time must have a good amount of saccades constantly going on, or just do the cat thing naturally (look at point A, it does not move, but all the rest starts moving, so look at point B, B now stops, but all the rest including A starts moving, etc)