<ul>
<li><img src="../img/long.png" alt=""></li>
<li><img src="../img/龙.jpg" alt=""></li>
</ul>
<script>
var lis=document.querySelectorAll("img");
for(let i in lis){
  lis[i].onClick=function () {
document.body.style.backgroundImage='url('+this.src+')'//将点击的图片路径给大盒子背景路径;
}};
</script>