var banArray = new Array(28); // increase size as necessary
var counter = 0;

// preload all banner images
if (document.images) {
	for(i = 0; i < 28; i++) {
		banArray[i] = new Image(240, 180);
		banArray[i].src = "images/midp" + (i) + ".jpg";
	}
}

// banner changer function
function changeBanner() {
	if (counter >= 28)
		counter = 0;
	document.banner.src = banArray[counter].src;
	counter += 1;
}
 
//calls the changeBanner() function every 4 seconds
//change the timer as necessary (minutes * 60000) or (seconds * 1000)
var timer = window.setInterval("changeBanner()", 3000);