var imagenumber = 11 ;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1 ;
images = new Array
images[1] = "images/ran_main01.gif"
images[2] = "images/ran_main02.gif"
images[3] = "images/ran_main03.gif"
images[4] = "images/ran_main04.gif"
images[5] = "images/ran_main05.gif"
images[6] = "images/ran_main06.gif"
images[7] = "images/ran_main07.gif"
images[8] = "images/ran_main08.gif"
images[9] = "images/ran_main09.gif"
images[10] = "images/ran_main10.gif"
images[11] = "images/ran_main11.gif"

var image = images[rand1]