banner1 = new Image();
banner1.src = "images/bio/takoSake.jpg";
banner2 = new Image();
banner2.src = "images/bio/slug.jpg";
banner3 = new Image();
banner3.src = "images/bio/tako.jpg";
banner4 = new Image();
banner4.src = "images/bio/dino.jpg";
banner5 = new Image();
banner5.src = "images/bio/bork.jpg";
banner6 = new Image();
banner6.src = "images/bio/radiation.jpg";
banner7 = new Image();
banner7.src = "images/bio/lightHomme.jpg";

function randombanner(){
var randomnumber = Math.random();
i = 7 // Number of banners that you have
i = Math.round( (i - 1) * randomnumber) + 1;
document.banner.src = eval("banner" + i + ".src");
}