getImg.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .result{
      width: 300px;
      height: 300px;
      background: #eee;
    }
    .result4 .demo {
      position: absolute;
      top: 50%;
      left: 50%;
      margin-top: -765px;
      margin-left: -765px;
      width: 1529px;
      height: 1529px;
      background: url(/light.png) no-repeat;
      background-size: 100% 100%;
    }
  </style>
</head>

<body>
  <div class="result" id="result">
    <div class="demo"></div>
  </div>
  <button onclick="fun()">点击加载图片</button>
  <script>
    function fun(){
      console.log('chufa ')
      result.className = 'result result4'
    }
  </script>
</body>

</html>