body:before{
  content: ' ';
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  z-index:-1;
  background: url(https://pic697.oss-cn-guangzhou.aliyuncs.com/main.jpg) center 0 no-repeat;
  background-size: contain;
}