function randomImage()
{
var imagesarray = new Array('images/header_01.png','images/header_02.png','images/header_03.png','images/header_04.png','images/header_05.png','images/header_06.png','images/header_07.png','images/header_08.png');
var randomnumber = Math.round(Math.random()*(imagesarray.length - 1));
document.images.headerpic.src = imagesarray[randomnumber];
}