function swapImg(event){
	var item = Event.element(event);
	$("top_image").src = "images/top_image_" + item.id + ".jpg";
}

function resetImg(){
	$("top_image").src = "images/top_image.jpg";
}

window.onload = function(){
	Event.observe($("mt"), "mouseover", swapImg, false);
	Event.observe($("mt"), "mouseout", resetImg, false);
	Event.observe($("sea"), "mouseover", swapImg, false);
	Event.observe($("sea"), "mouseout", resetImg, false);
}