codice:
<script language="javascript" type="text/javascript">
<!--

  if (document.all || document.layers || document.getElementById || window.opera)
  {
    var imageSrc = "";

    if (screen.width == 640 && screen.height == 480)
      imageSrc = "piccolo.jpg";

    else if (screen.width == 800 && screen.height == 600)
      imageSrc = "medio.jpg";

    else if (screen.width == 1024 && screen.height == 768)
      imageSrc = "grande.jpg";

    else
      imageSrc = "altro.jpg";

    document.writeln ("<body background=\"" + imageSrc + "\">");
  } else
    document.writeln ("Errore fatale: Il browser " + navigator.appName + " non supporta JavaScript1.2.");

//-->
</script>
lo metti prima di </head>