<!DOCTYPE html>
<html>
<body style="background:#f3f3f3 url('img_tree.png') no-repeat right top;">
<h1>Hello World!</h1>
<button type="button" onclick="myFunction()">Get background values</button>
<script>
function myFunction() {
alert(document.body.style.background);
}
</script>
</body>
</html>