On sol 55, Curiosity rolled up to a sand drift called "Rocknest." On sol 56, the rover rolled its right front wheel onto the drift's peak, locked the other five wheels, then "scuffed" the drift by rotating the right from wheel 30 degrees. Then it backed up and surveyed the result.
The image blinks back and forth between photos taken before the scuff, on sol 56, and after, on sol 57. The two images were taken under different lighting conditions and from different positions, so the alignment is not great.