I studied this in psychology class, the image is just the same and depends on what side of your brain is being used. Which will determine which way your mind tells you it's going. About 90% of people will see it going to the anti-clockwise .
If it was two images there would be a pre-determined time in which they switched....which this in not.
* POST UPDATE
I just put this image in my GIF Animator software, there is 34 different pictures. When I start on the top and scroll down it go's clockwise, when I scroll up it go's anti-clockwise? Interesting, it must mean that if your brain see's it from the top down or from the bottom up it will rotate different.