#container{
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
  width: 100%;
  max-width: 250px;
}

.moving{
  position: absolute;
  top: -100px;
  left: -100px;
}
#shells img{
  max-width: 15px;
}