var ctr = 14;
function lights() {
 switch(ctr) {
  case 0:
   document.images['animBullet0'].src = "green_bullet_swap.gif";
   document.images['animBullet1'].src = "green_bullet.gif";
   ctr = 14;
   break;
  case 14:
   document.images['animBullet14'].src = "green_bullet_swap.gif";
   document.images['animBullet0'].src = "green_bullet.gif";
   ctr--;
   break;
  default:
   document.images['animBullet'+ctr].src = "green_bullet_swap.gif";
   document.images['animBullet'+(ctr+1)].src = "green_bullet.gif";
   ctr--;
 }
}
function animate() {
 setInterval('lights()',500);
}