for( var name in document.images ){
	var img = document.images[name];
	if( img.src  && img.src.indexOf("_off") != -1 ){
		var onSrc = img.src.replace("_off", "_on");
		var onImg = new Image();
		onImg.src = onSrc;
		img.onmouseover = img_mouseover;
		img.onmouseout = img_mouseout;
	}
}
function img_mouseover(){
	this.src = this.src.replace("_off","_on");
}
function img_mouseout(){
	this.src = this.src.replace("_on","_off");
}