var backImage = new Array(); 

backImage[0] = "../green.gif";
backImage[1] = "../paperc.jpg";
backImage[2] = "../grey.gif";
backImage[3] = "../paperb.jpg" 
backImage[4] = "";

function changeBGImage(whichImage){
if (document.body){
document.body.background = backImage[whichImage];
}
}