预加载

var images = new Array()

            function preload() {
                for (i = 0; i < preload.arguments.length; i++) {
                    images[i] = new Image()
                    images[i].src = preload.arguments[i]
                }
            }

            function preloader() {
                if (document.images) {
                    preload("img/a1.png", "img/a2.png", 
                                "img/a3.png", "img/a4.png", "img/b1.png", 
                                 "img/b2.png", "img/b3.png","img/b4.png"
                    )

                }
            }

            function addLoadEvent(func) {
                var oldonload = window.onload;
                if (typeof window.onload != 'function') {
                    window.onload = func;
                } else {
                    window.onload = function() {
                        if (oldonload) {
                            oldonload();
                        }
                        func();
                    }
                }
            }
            addLoadEvent(preloader);

原文：http://web.jobbole.com/86785/