var NumBGImages = 20;
var randNum = Math.floor(Math.random() * NumBGImages);

var str = '' + randNum;  
while (str.length < 2) {  
  str = '0' + str;  
}  
randNum = str;  

document.write('<img src="images/rotate/pic' + randNum + '.jpg" alt="" border="2">');