If it doesn't work correctly you'll have to flush the content of your templates_c folder.
<SCRIPT LANGUAGE=javascript>
if (screen.height<=600) {document.write('<BODY background="imgages/fdacc800.jpg" leftmargin=0 topmargin=0 bgproperties=fixed>');};
if ((screen.height>600)&&(screen.height<=768)) {document.write('<BODY background="imgages/fdacc1024.jpg" leftmargin=0 topmargin=0 bgproperties=fixed>');};
if (screen.height>768) {document.write('<BODY background="images/fdacc1280.jpg" leftmargin=0 topmargin=0 bgproperties=fixed>');};
</SCRIPT>