could someone please tell help me with this code?

Basically I have 2 identical images one is shot in daylight where the other one is shot at night, I want it where the light one is the original image and as the page is scrolled down its starts to blend into the darker one.

```
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    <html xmlns="http://www.w3.org/1999/xhtml">
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>BG Test</title>

    <script>
    var start = 100 // how far the user has to scroll down before it start fading
    var end = 1200 // the number of pixels the user has to scroll down before the opacity is 0.

    $(document).scroll(function(){
        if(scrollTop>start){
            $('images/MainLight.png').css({'body':(end-scrollTop)/(end-start)});
        }
    });
    </script>
    </head>

    <style type="text/css">
    body{
        background:url(images/MainLight.png) no-repeat center center fixed;
        opacity:100;
        width:100%;
        z-index:-1;

    }

    bgimg{
        background:url(images/MainDark.png) no-repeat center center fixed;
        opacity:100;
        width:100%;
        z-index:0;
    }

    </style>

    <div class="body" >

       Top!
       <br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br                                 /><br /><br /><br /><br /><br /><br /><br /><br /><br />
        <br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />        <br /><br /><br /><br /><br /><br /><br /><br />
       Middle!
       <br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />        <br /><br /><br /><br /><br /><br /><br /><br />
               <br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br         /><br /><br /><br /><br /><br /><br /><br /><br /><br />
       Bottom!

    </div>

    <body>
    </body>
    </html>

```

Many Thanks in advance.