保证1920px的图片，在低分率率的电脑上也能正常显示，两边裁剪，中间居中，高度不变
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<div class="test">
<div style="width: 100%;height: 390px; background: url(http://imagecdn0.17huo.com/o_1bvue57hgbp8ms1diu18ctjnrh.jpg) no-repeat center;background-size: 1920px 390px;"></div>
</div>
</body>
</html>