Preloading the images creates a smoother transition between the two when the rollover is initiated. Because the browser requests preloaded images as the page loads, images are ready to display when needed. If images aren't preloaded, the browser doesn't request the file until the rollover is triggered. Without preloading, large images or slow Internet connections can cause considerable delays. When a visitor moves the pointer over the eye image, it begins to blink.
Animation continues as long as the pointer stays over it.
All a visitor has to do to stop the animation is move the pointer. Always start with images that are the same size or the browser will scale them to match, with sometimes bizarre results. Animation effects created with JavaScript won't work if users have JavaScript turned off in their browsers. A little animation goes a long way. Use it to provide a sense of movement on the page without inducing motion sickness.