function Banners()
{
    var MNews = new Array();

    MNews[0]= '<IMG SRC="images/topo.jpg"  WIDTH=548 HEIGHT=150 BORDER=0 align="center"></a>'

    MNews[1]= '<IMG SRC="images/topo2.jpg"  WIDTH=548 HEIGHT=150 BORDER=0 align="center"></a>'

    MNews[2]= '<IMG SRC="images/topo3.jpg"  WIDTH=548 HEIGHT=150 BORDER=0 align="center"></a>'


   var Numero = Math.floor(Math.random()*3);
   document.write(MNews[Numero]);
}

Veja_Banner = Banners();