var imagenumber = 3 ;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) +1;

images = new Array
images[1] = "<img src='images/midbanner_new.jpg' border=0>"
images[2] = "<img src='images/midbanner_new1.jpg' border=0>"
images[3] = "<img src='images/midbanner_new2.jpg' border=0>"
//images[4] = "<img src='images/midbanner-4.jpg' border=0>"
//images[5] = "<img src='images/midbanner-5.jpg' border=0>"
//images[6] = "<img src='images/midbanner-6.jpg' border=0>"

var image = images[rand1]