This is not an animation. Your eyes are making it move. If you don't believe me, print it out on paper. To test this illusion, stare at one spot for a few seconds and everything will stop moving. Or look at the black center of each circle and it will stop moving. But move your eyes to the next black center and the previous will move after you take your eyes away from it.
The white cubes are stacked vertically in parallel. Do they appear to lean to the right?
The purple box is a perfect square but it appears to bend inward.
The soldiers are all of the same height.
The blue cubes are the same color in both left and right images.
The red circles are both the same size.
The black and yellow bars are parallel and perpendicular, not staggered.
Which vertical red bar looks longer? They are actually the same length.
Stare at the negative image of Queen Elizabeth for about 30 seconds until you see the positive image on the right.
Afterimages are opposites so what was white is black, and what was black is white.
Is it a duck looking to the left or a rabbit looking to the right?
Where is the highest point?
With one eye covered, stare at the upper flag for about 30 seconds until you see the normal red, white and blue in the lower box.
The afterimage is a color "opposite." To understand why, you'll need to read about the properties of light and color.
The slight rotation and alternating color of the sqaures disrupt pattern formation, making it appear that the circles cross.