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

if (ad==1) {
banner="images/video2.jpg";
}

if (ad==2) {
banner="images/video3.jpg";
}

if (ad==3) {
banner="images/video4.jpg";
}

if (ad==4) {
banner="images/video1.jpg";
}

document.write('<img src="'+banner+'" width="265" height="426" border="0" align="right" valign="top"/>');