This will require a combination of CSS, javascript and as3.

You can communicate with your surrounding environment via as3's ExternalInterface class. Read more on it here.
http://help.adobe.com/en_US/FlashPla...Interface.html

Sorry I don't have time to verify the exact code but will look something like this.
You'll have a javascript function waiting to be called by your swf which will modify the css

So just before the end of you <head></head> tag you'd have

Code:
<script>
function changeBackground(myImg){
  document.body.style.background="background-image: url(' + myImg + ') no-repeat";
}
</script>
Then somewhere in your as3 you'd have
Code:
ExternalInterface.call("changeBackground", "image1.jpg");