prova così:

codice:
<script language="javascript" type="text/javascript">
  var intWid;
  intWid = screen.width;
  if (intWid <= 1024) document.body.style.backgroundImage = "url(/images/sfondo1024.jpg)";
  if (intWid > 1280) document.body.style.backgroundImage = "url(/images/sfondo1440.jpg)";
</script>
saluti