var bannerImages = new Array()

bannerImages[0] = 'images/sci-comm-top-1.png'
bannerImages[1] = 'images/sci-comm-top-2.png'
bannerImages[2] = 'images/sci-comm-top-3.png'
bannerImages[3] = 'images/sci-comm-top-4.png'

var p = bannerImages.length;
var whichImage = Math.round(Math.random()*(p-1));

function showBannerImage(){
  document.getElementById('bannerimage').src=bannerImages[whichImage];
}