Dropdown

<select id="monDropdown" onchange="updateDropdown">
  <option value="option1">Option1</option>
  <option value="option2">Option2</option>
  <option value="option3">Option3</option>
</select>

<img id="monImg" src="" alt="">
<div id="maDiv"></div>
<p id="monParagraphe"></p>

<script type="text/javascript">
  function updateDropdown() {
    if (document.getElementById("monDropdown").value == "option1"){
      document.getElementById("monImg").src = "lien/vers/monImg.png";

    }else if(document.getElementById("monDropdown").value == "option2"){
      document.getElementById("maDiv").innerHTML = "<h2>Mon titre</h2><p>Mon texte</p>";

    }else if(document.getElementById("monDropdown").value == "option3"){
      document.getElementById("monParagraphe").innerHTML = "Mon texte";
    }
  }
</script>