<!DOCTYPE html>
<html>
<head>
<style>
div {
width: 400px;
height: 400px;
border: 1px solid black;
}
</style>
</head>
<body>
<h1>Hello World!</h1>
<div id="myDiv">This is a div element.</div>
<br>
<button type="button" onclick="myFunction()">Set the background image of div</button>
<script>
function myFunction() {
document.getElementById("myDiv").style.backgroundImage = "url('img_tree.png')";
}
</script>
</body>
</html>