Untitled

<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script>

$(document).ready(function(){
  $(document).mousemove(function(e){
    $('img').css('left',e.pageX+"px");
    $('img').css('top',e.pageY+"px");
  });
});

function setBg(){
	var textBox = document.getElementById('tb').value;
	var myElement = document.querySelector("body");
	alert(textbox);
	myElement.style.backgroundImage = "url('" + textbox + "')";
}

</script>
<style>
img {
  position: absolute;
  transform: translate(-50%, -50%);
}

</style>

</head>
<body>
<input id="tb" type="text" onkeypress="setBg()" />
<img src="spotlight_bg.png" alt="" />
</body>
</html>