Are you watching closely as the image disappears?
All we ask you do is take a look at this image, and we mean really look. In fact, stare right into the centre of it. No games, just look and relax. See the image that you know is there disappear.
Done? Here's how it happened. You've just fallen victim to Troxler's fading. Hold your gaze at one particular point and your peripheral vision starts to disappear, the same sort of thing as you feel your t-shirt in the morning after putting it on, then don't feel it at all a few minutes after.
Neurons in your visual system become acquainted to the central focal point and begin to ignore the outside space, which turns it white.
The blurred image? Well that had to be used because something of a crisper definition would have had too much stimulus, meaning the trick wouldn't have worked, as your visual neurons would be constantly active with the other details. This would make it too difficult to maintain focus on the one particular are for the 20 seconds required for the fading to take place.