var ix = 0;
function ditherstep(){
    previous = ix - 1 ;
    if (previous == -1) previous = 6;
    document.images['arrow'+previous].className = 'plain';
    document.images['arrow'+ix].className = 'dithered';
    ix = (ix + 1) % 7;
    if (ix == 1){
        setTimeout('ditherstep()', 1500);
    }
    else{
        setTimeout('ditherstep()', 100);
    }
}

ditherstep();