If-Else 조건을 사용하여 JavaScript로 Div의 배경 이미지를 동적으로 변경하는 방법은 무엇입니까?

<code class="html"><div style="text-align:center;">
  <div class="ghor" id="a" onclick="chek_mark()"></div>
  function call
</div>
<script type="text/javascript">
  function chek_mark() {
    var el = document.getElementById("a").style.backgroundImage;
    if (el.url("Black-Wallpaper.jpg")) {
      el.url = "cross1.png";
    } else if (el.url("cross1.png")) {
      alert("<h1>This is working too.</h1>");
    }
  }
</script></code>

<code class="javascript">document.getElementById('a').style.backgroundImage="url(images/img.jpg)";</code>

<code class="css">.ghor {
    background-image: url('Black-Wallpaper.jpg');
    background-size: cover;
    border-radius: 5px;
    height: 100px;
    width: 100px;
    box-shadow: 2px 5px 7px 7px white;
    display:inline-block; 
}</code>