var swap_image = new Array;
swap_image[0]=new Image();
swap_image[0].src="images/arrow-l.png";
swap_image[1]=new Image();
swap_image[1].src="images/arrow-l-on.png";
swap_image[2]=new Image();
swap_image[2].src="images/arrow-r.png";
swap_image[3]=new Image();
swap_image[3].src="images/arrow-r-on.png";

function swapImage(id, img_idx) {
document.getElementById(id).src=swap_image[img_idx].src;
}