var imagenumber = 12 ;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1 ;
images = new Array
images[1] = "pictures/banner_1.jpg"
images[2] = "pictures/banner_2.jpg"
images[3] = "pictures/banner_3.jpg"
images[4] = "pictures/banner_4.jpg"
images[5] = "pictures/banner_5.jpg"
images[6] = "pictures/banner_6.jpg"
images[7] = "pictures/banner_7.jpg"
images[8] = "pictures/banner_8.jpg"
images[9] = "pictures/banner_9.jpg"
images[10] = "pictures/banner_10.jpg"
images[11] = "pictures/banner_11.jpg"
images[12] = "pictures/banner_12.jpg"
var image = images[rand1]
document.write('<IMG SRC="' + image + '">');