var arr_on = new Image();
    var arr_off = new Image();
    arr_on.src = "images/arrow_on.gif";
    arr_off.src = "images/arrow_r.gif";

    function _seOption(img_name){
       document.images[img_name].src = arr_on.src;
    }
    
    function _DeseOption(img_name){
       document.images[img_name].src = arr_off.src;
    }