var pathOfImg = "images/";
if (document.images)	{ 
	function buttonOn(imgName) {
	    document.images[imgName].src= pathOfImg + "nav_" + imgName + "_over.jpg"
	}
	  
	function buttonOff(imgName) {
	    document.images[imgName].src= pathOfImg + "nav_" + imgName + ".jpg"
	}
}