SOURCE

  window.onload = function() {
            document.getElementById('btn').onclick = function() {
                var input1 = document.getElementById("input1");
                var show = document.getElementById("show");

                if (input1.value == 1) {
                    show.innerHTML = "<span>折扣%10<span> <img src = 'https://i.loli.net/2020/04/09/IqPJrzvYUNRyEnc.jpg' > ";
                } else if (input1.value == 2) {
                    show.innerHTML = "<span>折扣%30<span> <img src = 'https://i.loli.net/2020/04/09/cYPMgfD1sFNtzGE.jpg' > ";
                } else if (input1.value == 3) {
                    show.innerHTML = "<span>折扣%50<span> <img src = 'https://i.loli.net/2020/04/09/3McLy1ST2qWPr9h.jpg' > ";
                } else {
                    show.innerHTML = "<span>请输入数字1,2,3</span>";
                }
            }
        }

 <h1>欢迎进入Javascript！</h1>
    <div id="show"></div>
    <div>
        <span>请输入一个数字 </span>
        <span><input type="text" id="input1"></span>
        <span><input  id="btn" type="button" value="提交" ></span>
    </div>

  #show {
            width: 400px;
            height: 200px;
            background-color: rgb(92, 219, 113);
        }
        
        div {
            border: 1px solid black;
            width: 400px;
            margin-bottom: 10px;
            padding: 30px;
        }