aon = new Image();
 aon.src = "mapa/zoom.gif";
 aoff = new Image();
 aoff.src = "mapa/mapa_as.jpg";
 bon = new Image();
 bon.src = "mapa/zoom.gif";
 boff = new Image();
 boff.src = "mapa/mapa_bs.jpg";
 con = new Image();
 con.src = "mapa/zoom.gif";
 coff = new Image();
 coff.src = "mapa/mapa_cs.jpg";
 don = new Image();
 don.src = "mapa/zoom.gif";
 doff = new Image();
 doff.src = "mapa/mapa_ds.jpg";

 function rollon(imgName) {
   document[imgName].src = eval(imgName + "on.src");
 }
 function rolloff(imgName) {
   document[imgName].src = eval(imgName + "off.src");
 }