var randnum = Math.random();
    var inum = 4;
    var rand1 = Math.round(randnum * (inum-1)) + 1;
    var images;
    images = new Array
    images[1] = "maincontentheader"
    images[2] = "maincontentheader1"
    images[3] = "maincontentheader2"
    images[4] = "maincontentheader3"
    var image = images[rand1]