var NumberOfAds = 4;
var now = new Date()
var sec = now.getSeconds()
var AdNumber = sec % NumberOfAds;
AdNumber +=1;
if (AdNumber==1)  Graphic="images/bigradio1.jpg";
if (AdNumber==2)  Graphic="images/bigradio2.jpg";
if (AdNumber==3)  Graphic="images/bigradio3.jpg";
if (AdNumber==4)  Graphic="images/bigradio4.jpg";
document.write('<IMG SRC="' + Graphic + '" WIDTH=390 HEIGHT=275 BORDER=0 ALT="Radio">')