#instagram-feed {
  width: 100%; 
  max-width: 600px; 
  float: left; 
}

#instagram-feed .instagram-image {
  width: 25%; 
  float: left;
  padding: 5px;
  -webkit-box-sizing: border-box; 
  -moz-box-sizing: border-box; 
  box-sizing: border-box; 
}

#instagram-feed .instagram-image img {
  display: none; 
}

#instagram-feed .instagram-image a {
  display: block; 
  background-color: #000;
  background-repeat: no-repeat;
  background-position: center;
  -webkit-background-size: cover; 
  -moz-background-size: cover; 
  -o-background-size: cover; 
  background-size: cover; 
}

#instagram-feed .instagram-image a::before { 
  content: ''; 
  display: block; 
  padding-top: 100%; 
}