var pN = 9 //-> qN = number of pics in archives (remember, in JS, 0 reads as 1)
var n = pN - 1
var x = Math.round( Math.random() * n )

document.write("<style type='text/css'>");
document.write("<!--");
document.write("#randbg {background-image: url(background/"+x+".jpg)}");
document.write("-->");
document.write("</style>");