#gallery div {
  width: 100%;
  margin: 0 auto;
 
}

#gallery ul {
  list-style-type: none;
}

#gallery li img {
  float: left;
  margin: 10px;
  border: 1px solid grey;

  -webkit-transition: box-shadow 0.5s ease;
  -moz-transition: box-shadow 0.5s ease;
  -o-transition: box-shadow 0.5s ease;
  -ms-transition: box-shadow 0.5s ease;
  transition: box-shadow 0.5s ease;
}

#gallery li img:hover {
  -webkit-box-shadow: 0px 0px 10px rgba(0,0,0,0.5);
  box-shadow: 0px 0px 10px rgba(0,0,0,0.5);
}