Solo per dare un' idea dello Straystudio pensiero :

commenta con // la riga dell' alert
//alert(document.getElementById("area").style.margin Left+" \nL = "+L);
codice:
<html>
<head>
<script language="JavaScript"> 
<!--
var L = 0;

function moveRight(){
 L++;
 document.getElementById("area").style.marginLeft = L +"px"; 
 alert(document.getElementById("area").style.marginLeft+" \nL = "+L);
 }

// -->
</script>

</head>
<body>

<input value="=>" type="button" onClick='moveRight();'>


<div style="height: 100%; border: 1px solid black; 
     background-image: url(http://www.dija.it/public/tyler.jpg); background-repeat: no-repeat;">

 <div id="area" style="width: 270; height: 294; border: 2px dashed red;"></div>

</div>

</body>
</html>
Puoi recuoerare L come Integer o tenere il valore in pixel.