Actually, this might be one of the intended effects of this feature. What better way to make it appear their catalog is wider and ever-changing then to cycle through these static stills every so often. It may not be the most user friendly design pattern to 'trick' your user into thinking there's a bigger catalog, but I can understand the desire for it from their perspective.

And to be perfectly honest, sometimes those changing stills have actually made me watch a movie that I had previously glossed over because the image wasn't as interesting. So from personal experience I can say their primary intent was reached w/ the periodic cycling of stills.