var imagenumber = 6 ;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1 ;

images = new Array
images[1] = "images/im_main1.jpg"
images[2] = "images/im_main2.jpg"
images[3] = "images/im_main3.jpg"
images[4] = "images/im_main4.jpg"
images[5] = "images/im_main5.jpg"
images[6] = "images/im_main6.jpg"
var image = images[rand1]