$(document).ready(function() {
var randomImages = ['rightColTop1', 'rightColTop2', 'rightColTop3', 'rightColTop4'];
    var rndNum = Math.floor(Math.random() * randomImages.length);
    $("div.topBannerRight").css({ background: "url(/Images/" + randomImages[rndNum] + ".gif) 100% 100% no-repeat" });    
});