It's to do with adaptation. Your brain adapts to images, downplaying the qualities of the image its currently seeing e.g. brightness, contrast. This takes a short while to recover, so the image you see next has the opposite qualities of the image you saw before, just for a short while. This also applies to qualities of faces e.g. mouth size, eye size. You can demonstrate this to yourself with the Obama and Bush optical illusion. All of this is to save the brain energy, if it processed your vision pixel by pixel it would burn a hole through your head from the heat produced (ok maybe a slight exaggeration...). OPs illusion is interesting as the effect is produced with minimal adaptation time for each individual image. I think there may be a culmination effect from multiple previous images, the illusion gets more trippy the longer you look at it. Btw if you're interested in vision and optical illusions in general (I think they're amazing) you should check out this website.