function bannerSide() {
};

bannerSide = new bannerSide();
numberSide = 0;

// bannerSideArray
bannerSide[numberSide++] = "<a href='http://www.galaxiworld.com' target='_blank'><img src='http://www.ivegas.com/images/ad_gw_1.gif' border='0'></a>"
bannerSide[numberSide++] = "<a href='http://www.galaxiworld.com' target='_blank'><img src='http://www.ivegas.com/images/ad_gw_2.gif' border='0'></a>"
// keep adding items here...

incrementSide = Math.floor(Math.random() * numberSide);

document.write(bannerSide[incrementSide]);