var imagenumber = 8 ;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1 ;

images = new Array;
images[1] = 'images/headers/header1.jpg';
images[2] = 'images/headers/header2.jpg';
images[3] = 'images/headers/header3.jpg';
images[4] = 'images/headers/header4.jpg';
images[5] = 'images/headers/header5.jpg';
images[6] = 'images/headers/header6.jpg';
images[7] = 'images/headers/header7.jpg';
images[8] = 'images/headers/header8.jpg';


var image = images[rand1];
document.write("<img src='" +image+ "' alt='' />");