原理：大图加载完成覆盖小图放大图。
<img src="img-small.jpg" width="960" height="1200" id="myImage" />
<script language="JavaScript" type="text/javascript">
var img = new Image();
img.src = "img-big.jpg";
img.onload = function() {
document.getElementById('myImage').src = this.src;
}
</script>
