Try it Editor: Select a value opacity

Edit This Code :

<!DOCTYPE html>
<html>
<body>

<p id="p1"><img src="http://www.dwfaisalabad.com/wp-content/uploads/2017/08/DWF.png" alt="" width="333" height="333" /></p>

<select onchange="myFunction(this);" size="7">
  <option>0
  <option>0.2
  <option>0.5
  <option>0.8
  <option selected="selected">1
</select>

<script>
function myFunction(x) {
// Return the text of the selected option
    var opacity = x.options[x.selectedIndex].text;
    var el = document.getElementById("p1");
    if (el.style.opacity !== undefined) {
        el.style.opacity = opacity;
    } else {
        alert("Your browser doesn't support this example!");
    }
}
</script>

</body>
</html>
Result :