I have done this javascript function: it fix your background image to your screen size in base at the most significative dimension (width od height) without change the image aspect ratio.
<script language="Javascript">
function FixBackgroundToScreen()
{
bgImg = new Image();
bgImg.src = document.body.background;
if ((bgImg.height / window.innerHeight) < (bgImg.width / window.innerWidth))
document.body.style.backgroundSize = "auto 100%";
else
document.body.style.backgroundSize = "100% auto";
};
</script>
This function is the only think you need. It must be called with the window.onresize event and from the body.onload event. The image must be background-repeat: no-repeat; background-attachment: fixed;
<script language="Javascript">
window.onresize=function(){FixBackgroundToScreen();};
</script>
<body onload="FixBackgroundToScreen();">
You can see the function in my site www.andreamarulla.it
Sorry for my english... Andrea ;-)