.photo-wall {
      
  display: flex;
  flex-wrap: wrap;
  
}

.photo-div {
  z-index: 0;

  height: 50vmin;
  width: 20%;



  background-color: rgba(128, 128, 128, 0.7);

  flex: 1;

  display: flex;

}

.photo-div.first-child{

}

.photo{
  flex: 1;
  width: auto;
  height: 100%;
}