body:before {
    background: url(https://images.cnblogs.com/cnblogs_com/moniter/1689406/o_200404074125003.png) center/cover no-repeat;
    content: '';
    background-repeat: no-repeat;
    background-position: center;
    opacity: 0.8;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
    -webkit-filter: grayscale(70%);
    -moz-filter: grayscale(70%);
    -ms-filter: grayscale(70%);
    -o-filter: grayscale(70%);
    filter: grayscale(70%);
    filter: gray;
}

<!-- 图片放大-->
#mainContent img
{
    z-index: 1000; //设置为最顶层，以免左侧导航栏遮挡
    position: relative;
    cursor: pointer;//鼠标样式：手势
    transition: all 1.2s; //放大的时间：1.2s
}