while(true){
aleatorio = Math.random();
numero = Math.round(aleatorio * 10);

if (numero == 0){
imagen = "http://img23.imageshack.us/img23/866/bannerstation01.jpg";
break;}

if (numero == 1){
imagen = "http://img23.imageshack.us/img23/723/logo10.png";
break;}

if (numero == 2){
imagen = "http://img23.imageshack.us/img23/7048/ninjabanner001.jpg";
break;}

}
document.write('<a href="http://sector-b.foroactivo.net/portal.htm" target="_self" ><img src="' + imagen + '" border=0 style="width: 1000px; height: 300px"></a>');