vyska = screen.height;
body = document.getElementsByTagName('body')[0].style;
if (vyska <= 800) body.backgroundImage = "url(images/background-800.jpg)";
else if (vyska <= 1024) body.backgroundImage = "url(images/background-1024.jpg)";
else if (vyska <= 1200) body.backgroundImage = "url(images/background-1200.jpg)";
else if (vyska <= 1600) body.backgroundImage = "url(images/background-1600.jpg)";