var imagenumber = 13 ;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1 ;

images = new Array
images[1] = "mainheader01.jpg"
images[2] = "mainheader02.jpg"
images[3] = "mainheader03.jpg"
images[4] = "mainheader04.jpg"
images[5] = "mainheader05.jpg"
images[6] = "mainheader06.jpg"
images[7] = "mainheader07.jpg"
images[8] = "mainheader08.jpg"
images[9] = "mainheader09.jpg"
images[10] = "mainheader10.jpg"
images[11] = "mainheader11.jpg"
images[12] = "mainheader12.jpg"
images[13] = "mainheader13.jpg"

var image = images[rand1]