var myArray = new Array()

myArray[0]='images/header1.jpg';
myArray[1]='images/header2.jpg';
myArray[2]='images/header3.jpg';
myArray[3]='images/header4.jpg';
myArray[4]='images/header5.jpg';
myArray[5]='images/header6.jpg';
myArray[6]='images/header7.jpg';
myArray[7]='images/header8.jpg';
myArray[8]='images/header9.jpg';
myArray[9]='images/header10.jpg';
myArray[10]='images/header11.jpg';

var s = Math.floor(Math.random() * myArray.length);
document.write('<style type="text/css">#header { height: 212px; text-align: left; background: url(' + myArray[s]  + ') no-repeat; }</style>');