window.onload=function(){  
    window.img=document.getElementById("bg-img");  
    window.hi=window.img.height;
    window.wi=window.img.width
	
		    		
    resizeImage();
}
window.onresize=function(){
    resizeImage();
}

function resizeImage(){
	
	h=document.body.clientHeight;
	f=document.body.clientHeight/window.hi;
	w=window.wi*f;
	
	if(w<document.body.clientWidth) f=document.body.clientWidth/w;
	else f=1;
	
	w=w*f;
	h=h*f;
	
	if(h>document.body.clientHeight) offset=h-document.body.clientHeight;
	else offset=0;
	

	
	window.img.height=h;
	window.img.width=w;
	window.img.style.top=-offset;
	

}