图片预加载即提前加载图片，可保证图片快速、无缝地发布，用户需要查看时可直接从本地缓存中渲染，适用于图片占据很大比例的网站。

方法1,在CSS background中加载：会增加页面的整体加载时间

#preload-01 { background: url(-01.png) no-repeat -9999px -9999px; }
#preload-02 { background: url(-02.png) no-repeat -9999px -9999px; }

方法2，JS new image对象，设置src加载：

function preloader() {
    if (document.images) {
        var img1 = new Image();
        var img2 = new Image();
        var img3 = new Image();
        img1.src = ";;
        img2.src = ";;
        img3.src = ";;
    }
}
function addLoadEvent(func) {
    var oldonload = window.onload;
    if (typeof window.onload != 'function') {
        window.onload = func;
    } else {//onload中未挂载函数才执行该JS
        window.onload = function() {
            if (oldonload) {
                oldonload();
            }
            func();
        }
    }
}
addLoadEvent(preloader);
div.appendChild(img1);//插入到DOM

方法3，Ajax预加载,new Image()对象设置src

window.onload = function() {
    setTimeout(function() {
        // XHR to request a JS and a CSS
        var xhr = new XMLHttpRequest();
        xhr.open('GET', url;);
        xhr.send();
        xhr = new XMLHttpRequest();
        xhr.open('GET', url;);
        xhr.send();
        // preload image
        new Image().src = ";;
    }, 1000);
};

懒加载：将图片src赋值为一张默认图片，当用户滚动滚动条到可视区域图片时候，再去加载真正的图片

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Lazyload 2</title>
    <style>
    img {
        display: block;
        margin-bottom: 50px;
        height: 200px;
    }
    </style>
</head>
<body>
    <img src="images/loading.gif" data-src="images/1.png">
    <img src="images/loading.gif" data-src="images/2.png">
    <img src="images/loading.gif" data-src="images/3.png">
    <img src="images/loading.gif" data-src="images/4.png">
    <img src="images/loading.gif" data-src="images/5.png">
    <img src="images/loading.gif" data-src="images/6.png">
    <img src="images/loading.gif" data-src="images/7.png">
    <img src="images/loading.gif" data-src="images/8.png">
    <img src="images/loading.gif" data-src="images/9.png">
    <img src="images/loading.gif" data-src="images/10.png">
    <img src="images/loading.gif" data-src="images/11.png">
    <img src="images/loading.gif" data-src="images/12.png">
    <script>

    function throttle(fn, delay, atleast) {//函数绑定在 scroll 事件上，当页面滚动时，避免函数被高频触发，
        var timeout = null,//进行去抖处理
        startTime = new Date();
        return function() {
        var curTime = new Date();
        clearTimeout(timeout);
        if(curTime - startTime >= atleast) {
            fn();
            startTime = curTime;
        }else {
            timeout = setTimeout(fn, delay);
        }
        }
    }
    function lazyload() {
        var images = document.getElementsByTagName('img');
        var len    = images.length;
        var n      = 0;      //存储图片加载到的位置，避免每次都从第一张图片开始遍历        
        return function() {
        var seeHeight = document.documentElement.clientHeight;
        var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
        for(var i = n; i < len; i++) {
            if(images[i].offsetTop < seeHeight + scrollTop) {
                if(images[i].getAttribute('src') === 'images/loading.gif') {
                 images[i].src = images[i].getAttribute('data-src');
                }
            n = n + 1;
             }
        }
        }
    }
    var loadImages = lazyload();
    loadImages();          //初始化首页的页面图片
    window.addEventListener('scroll', throttle(loadImages, 500, 1000), false);
　　//函数节流（throttle）与函数去抖（debounce）处理,
//500ms 的延迟，和 1000ms 的间隔，当超过 1000ms 未触发该函数，则立即执行该函数，不然则延迟 500ms 执行该函数
    </script>
</body>
</html>