body{
  margin: 10px 4px;
}
.my-gallery{
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
}
.item{
  margin: 1px;
}
.pic-a{
  display: block;
  background-color: #AAA;
  width: calc(25vw - 4px);
  height: calc(25vw - 4px);
  overflow: hidden;
  background-size: cover;
  background-position: center;
}