background positioning not working

[ketanco]

I can not adjust the position of the main background for my webpage. The website is robotdukkani.com. I wanted to move the main blue background to 100px left, so here is my css. but, whatever value I write to top or left, it does not move the position of the blue background. How can I make the main blue background to start where I want?
.mainarea
{
z-index:-6;
position:absolute;
top:0;
left:100;
width:900px;
height:1300px;
}
and... here is the html on my index page.
<div class="mainarea">

<img src="pictures/mainbackgr.jpg" width="100%" height="100%">

</div>

[themarty]

http://www.w3schools.com/css/pr_background-position.asp

[ketanco]

ok...thanks for the answer but I discovered the mistake in my code. i just forgot to put px values to top and left...:) and ...yours work too...