function move_fixed_background() {
	var v
	if( document.body.clientWidth <= 768 )
		v = 'url(images/background_800.jpg)';
		
	else if( document.body.clientWidth >= 768 && document.body.clientWidth <= 1024)
		v = 'url(images/background_1024.jpg)';
		else if( document.body.clientWidth > 1024 )
		v = 'url(images/background_1280.jpg)';
	if( document.body.style.backgroundImage != v )
		document.body.style.backgroundImage = v;
}