分三层，第一层relative,第二层absolute，第三层fixed
扩充父元素的宽高

• html

<div class="page-container">
    <div class="front-container">
        <div class="front-bg">
            <img src="img/01_02.jpg" class="bg">
        </div>
     </div>
</div>

• css

.page-container{
    position: relative;
}
.front-container{
    min-height: 690px;         
    max-height: 750px;         
    position: absolute;
    right: 0;
    top: 0;
    bottom: 0;          
    left: 0;
}
.front-bg {
    position: fixed;
    width: 200%;
    height: 200%;
    left: -50%;
    background: #fff;
}
.front-bg img {
    display: block;
    margin: auto;
    min-width: 50%;
    min-height: 50%;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
}