PHP-Code:
<SCRIPT LANGUAGE="JavaScript"> 
<!-- 


var breite = screen.width; 

if (breite >= 1280) 
document.write('<body style="background-image:url(grafik/kugel1280.JPG); background-repeat:no-repeat; background-attachment:fixed; ">'); 
else 
if (breite >= 1024) 
document.write('<body style="background-image:url(grafik/kugel.JPG); background-repeat:no-repeat; background-attachment:fixed; ">'); 
else 
if (breite <= 800) 
document.write('<body style="background-image:url(grafik/kugel800.JPG); background-repeat:no-repeat; background-attachment:fixed; ">'); 
//--> 
</script> 
Ganz simpel ... so gehts natürlich auch