Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js">
</script>
<script>
$(document).ready(function(){
  $("button").click(function(){
    $("img").attr("width","500");
  });
});
</script>
</head>
<body>

<img src="img_pulpitrock.jpg" alt="Pulpit Rock" width="284" height="213">
<br>
<button>Set the width attribute of the image</button>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.