How to center my entire webpage including the background

Question 1

Try:

#container{
  width:960px;
  margin:0 auto;
  /*more CSS*/
}

You can change 0 to whatever, like 50px. Your image width is larger than 960px though.

Maybe you want something like this:

var pre = onload; // window is implicit
onload = function(){ // I personally don't indent directly inside the onload
if(pre)pre(); // execute old window.onload if it existed
var doc = document, IE = parseFloat(navigator.appVersion.split('MSIE')[1]);
function E(e){
  return doc.getElementById(e);
}
function alignTop(id){
  var e = E(id), w, h;
  if(IE){
    h = parseInt(e.currentStyle.height);
  }
  else{
    h = parseInt(getComputedStyle(e).getPropertyValue('height'));
  }
  w = innerHeight || doc.documentElement.clientHeight || doc.body.clientHeight;
  e.style.marginTop = w/2-h/2+'px';
}
alignTop('container');
}

You should put your script tag in the head so the body is defined in some older Browsers. I would use external JavaScript, so it's cached, like:

<!DOCTYPE html>
<html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en' lang='en'>
  <head>
    <title>Your Title Here</title>
    <meta http-equiv='content-type' content='text/html;charset=utf-8' />
    <meta name='keywords' content='some words to help SEO here' />
    <style type='text/css'>
      @import 'common.css'; @import 'thisPage.css';
    </style>
    <script type='text/javascript' scr='someName.js'></script>
  </head>
<body>
  <div>
    <div id='container'>Example Only</div>
  </div>
</body>
</html>

To see a working model, visit http://jsfiddle.net/MeMQz/2/ .

Question 2

i know that centers it but i want the top to be equal to the bottom by centering the whole page.not just left and right – HeyItsProdigy

Based on your comment above I guess you are trying to make the height of the content is 100% equal based on the height of the browser. You can use this trick:

html, body{
   height: 100%;
}
#container{
    display: block;
    height: 100%;
}

Have a look at the result here http://jsfiddle.net/qiqiabaziz/GB4W2/1/