function roll_on() {
	this.src = this.src.replace(/_off/,'_on');
}

function roll_off() {
	this.src = this.src.replace(/_on/,'_off');
}

function img_test() {
	for (i = 0; i < document.images.length; i++) {
		if (document.images[i].src.indexOf('_off.') > 0) {
			document.images[i].onmouseover = roll_on;
			document.images[i].onmouseout = roll_off;
		}
	}
}

img_test();