If you use JavaScript rollover images on your Web pages, it’s a good idea to preload them in order to reduce lag time while the image downloads. Non-preloaded images won’t show up immediately when the user rolls over the images in question, and if the user has a slow connection and/or the rollover images are large, they may take so long to appear that the purpose is defeated.

Images can be preloaded by creating a new image object and specifying its source. In order to be polite to browsers that don’t support the JavaScript document.images array, provide an if statement at the top: