var $_file = document.getElementById("files");
var img_box = document.getElementById("img_box");
$_file.addEventListener('change',function () {
var file = $_file.files[0];
var readerFile = new FileReader();
readerFile.onload = function (ev) {
var data = ev.target.result;
img_box.style.backgroundImage = 'url(' +data+ ')';
}
readerFile.readAsDataURL(file);
})
HTML5新增属性，简单演示，等闲下来在细究。
