nada, tengo esto
<script language="JavaScript">
if (screen.width + "x" +screen.height == "800x600") document.body.style.backgroundImage = "url(fondo8.jpg)";
if (screen.width + "x" +screen.height == "1024x768") document.body.style.backgroundImage = "url(fondo10.jpg)";
if (screen.width + "x" +screen.height == "1280x1024") document.body.style.backgroundImage = "url(fondo12.jpg)";
if (screen.width + "x" +screen.height == "1600x1200") document.body.style.backgroundImage = "url(fondo16.jpg)";
</script>
y no funciona, podeís verlo aqui:
www.informagestudios.com/web.html.
Las imágenes estan en el mismo directorio que el html...