点击图片body就变成图片  

修改元素属性 src     1.获取元素

var one = document.getElementById("one");
    var tu = document.getElementById("tu");
    var img = document.querySelector("img");
    one.onclick = function () {
      img.src = "/img/images/1.jpg";
      img.title = "第一张";
    };
    tu.onclick = function () {
      img.src = "/img/images/3.jpg";
      img.title = "第二张";
    };