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: lightcoral;
    }
    
    div {
        border: 1px solid black;
        width: 400px;
        margin-bottom: 10px;
        padding: 30px;
    }