// set the number of images to choose from
imageGroup1 = new Array(4);

// set the image files to choose from
imageGroup1[0] = 'banner_davidg.jpg';
imageGroup1[1] = 'banner_martin.jpg';
imageGroup1[2] = 'banner_olivier.jpg';
imageGroup1[3] = 'banner_zeller.jpg';

// choose the image
index1 = Math.floor(Math.random() * imageGroup1.length);