Função Trocar Peças

Teste realizado para um projeto futuro em meu site.

HTML

<style>
      a {
        color: transparent;
        padding: 40px;
        background-image: url(http://www.gilmardesigner.com/Imagens/social/instagram.png);
        background-size: contain;
        background-position: center;
        background-repeat: no-repeat;
      }
      
      li {
        list-style: none;
        margin-top: 85px;
      }

    </style>
    <script>
      function Twitter(tag) {
        var tag_li = document.getElementById('lista_menu');
        var tag_a = tag_li.getElementsByTagName('a');
        for (i = 0; i < tag_a.length; i++) {
          tag_a[i].style.backgroundImage = "mesmo";
        }
        tag.style.backgroundImage = "url(http://gilmardesigner.com/Imagens/social/twitter.png)"; // altera o fundo
      }
      function fb(tag) {
        var tag_li = document.getElementById('lista_menu');
        var tag_a = tag_li.getElementsByTagName('a');
        for (i = 0; i < tag_a.length; i++) {
          tag_a[i].style.backgroundImage = "mesmo";
        }
        tag.style.backgroundImage = "url(http://gilmardesigner.com/Imagens/social/fb.png)"; // altera o fundo
      }
      function email(tag) {
        var tag_li = document.getElementById('lista_menu');
        var tag_a = tag_li.getElementsByTagName('a');
        for (i = 0; i < tag_a.length; i++) {
          tag_a[i].style.backgroundImage = "mesmo";
        }
        tag.style.backgroundImage = "url(http://gilmardesigner.com/Imagens/social/email.png)"; // altera o fundo
      }

    </script>
    
  <body>
    <ul id="lista_menu">
      <li><a href="#" onclick="Twitter(this);">1</a></li>
      <li><a href="#" onclick="fb(this);">2</a></li>
      <li><a href="#" onclick="email(this);">3</a></li>
      <li><a href="#" onclick="email(this);">4</a></li>
    </ul>
  </body>