.reference  {
    width: 100%;
    height: 100px;
    margin: 2em auto;
    overflow: hidden;
    }
.reference ul {
    width: 100%;
    height: 100%;
    padding:0; margin:0;
    list-style:none;
    -webkit-display:inline-flex;
    -moz-display:inline-flex;
    display:inline-flex;
    justify-content: space-between;
    }

.reference img {
    width: 100%;
    height:100%;
  }

@media (max-width: 767px) {
 .reference {
   height: 100%;
   overflow:visible;
 }
 .reference ul {
    flex-wrap: wrap;
     justify-content: space-around;
  }
  .reference ul li{
    margin-bottom:5px;
  }
}