<!DOCTYPE html>
<html>
<body>
<h1>Hello World!</h1>
<button type="button" onclick="myFunction()">Set background image</button>
<script>
function myFunction() {
document.body.style.backgroundColor = "#f3f3f3";
document.body.style.backgroundImage = "url('img_tree.png')";
}
</script>
</body>
</html>