function imageOff(imgName) {
var buttonOff = new Image();
buttonOff.src = "./images/btn_" + imgName +"_off.gif";

	if(document.images)	{
	   	  	document[imgName].src = buttonOff.src;
	}	
}

function imageOn(imgName) {
var buttonOn = new Image();
buttonOn.src = "images/btn_" + imgName + "_on.gif";
	if(document.images)	{
		document[imgName].src = buttonOn.src;
	}	
}