// Function that return a number between 0 and "nums"
function getRandom(nums)
{
    var ranNum= Math.round(Math.random()*nums);
    return ranNum;
}

// Tells us how many images we have available.
var numberOfImages  = 2;
var randomNumber    = getRandom(numberOfImages-1);

// Create an array to hold the names of all images.
var image = new Array(numberOfImages);
image[0]="banner_5.jpg";
image[1]="banner_6.jpg";

// Write the img tag with a random image name.
document.write("<img alt='Qvault Vermont Images' height='158' width='391' src='/images/banners/" + image[randomNumber] + "' />");