为什么报错，运行不了

```javascript
//获取按钮的id
    var top = document.getElementById("top");
    var below = document.getElementById("below");

    //获取img图片
    var img = document.getElementsByTagName("img")[0];

//创建一个数组保存图片
    var picture = ["pexels-eberhard-grossgasteiger-1624255.jpg","pexels-helena-lopes-841303.jpg","pexels-iain-2350366.jpg","pexels-nick-wehrli-3762284.jpg","pexels-peter-spencer-663487.jpg","pexels-roberto-nickson-2609463.jpg","pexels-s-migaj-747964.jpg","pexels-stein-egil-liland-1933318.jpg"];

//创建一个变量保存索引
    var index = 0;

//点击按钮后事件的发生
    top.onclick = function () {
      img.src = picture[index];
      index--;
    };
    below.src = function () {
      img.src = picture[index]  ;
      index++;
    };

![img](https://img-mid.csdnimg.cn/release/static/image/mid/ask/693073563926144.png 'image.png')

你这很显然是没有获取到这个图片资源.
img为null,
设置 img.src的时候才会报Cannot set property 'src' of null错误

你可以看控制台错误的提示，另外below.src = function () { 这个写法对吗？

报的什么错啊

below是什么标签啊只有src 、srcipt又src属性