var rollover_OK 	= document.images;
var rollover_on		= new Object();
var rollover_off 	= new Object();

// define the "on" file for the image and preload it
function rollover(img,file) {
	if (rollover_OK) {
		var tmp = new Image();
		tmp.src = file;
		rollover_on[img] = tmp.src;
	}
}

// swap image source to "on" file
function rollon(img) {
	if (rollover_OK && document.images[img]) {
		rollover_off[img] = document.images[img].src;
		if (rollover_on[img]) document.images[img].src = rollover_on[img];
	}
}

// swap image source to "off" file
function rolloff(img) {
	if (rollover_OK && document.images[img] && rollover_off[img]) {
		document.images[img].src = rollover_off[img];
	}
}

rollover('thru1','images/blue.jpg')
rollover('thru2','images/blue.jpg')
rollover('thru3','images/blue.jpg')
rollover('thru4','images/blue.jpg')
rollover('thru5','images/blue.jpg')
rollover('thru6','images/blue.jpg')
rollover('thru7','images/blue.jpg')
rollover('thru8','images/blue.jpg')
rollover('thru9','images/blue.jpg')
rollover('thru10','images/blue.jpg')
rollover('thru11','images/blue.jpg')
rollover('thru12','images/blue.jpg')