var how_many_ads = 4;
var now = new Date();
var sec = now.getSeconds();
var ad = sec % how_many_ads; 
ad +=1;

if (ad==1) {
banner="images/stk_04a.jpg";
}

if (ad==2) {
banner="images/stk_04b.jpg";
}

if (ad==3) {
banner="images/stk_04c.jpg";
}

if (ad==4) {
banner="images/stk_04d.jpg";
}

document.write('<img src=' + banner + ' width=506 height=285 alt=Santika Indonesia border=0>');