function banner() {
};

banner = new banner();
number = 0;

// bannerArray
banner[number++] = "<a href='http://www.rondjenoordereiland.nl/'><img src='http://www.rondjenoordereiland.nl/images/banner/bannerlinks/finish.jpg' border='0'></a>"
banner[number++] = "<a href='http://www.rondjenoordereiland.nl/'><img src='http://www.rondjenoordereiland.nl/images/banner/bannerlinks/blusboot.gif' border='0'></a>"
banner[number++] = "<a href='http://www.rondjenoordereiland.nl/'><img src='http://www.rondjenoordereiland.nl/images/banner/bannerlinks/kanos.gif' border='0'></a>"
banner[number++] = "<a href='http://www.rondjenoordereiland.nl/'><img src='http://www.rondjenoordereiland.nl/images/banner/bannerlinks/poster2010.gif' border='0'></a>"
banner[number++] = "<a href='http://www.rondjenoordereiland.nl/'><img src='http://www.rondjenoordereiland.nl/images/banner/bannerlinks/willemsbrug.gif' border='0'></a>"
// keep adding items here...

increment = Math.floor(Math.random() * number);

document.write(banner[increment]);