.thumb {
  display: block;
  position: relative;
  font-size: 0;
  height: auto !important;
}

.thumb img {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  max-width: 100%;
  max-height: 100%;
  margin: auto;
}

.thumb::after {
  content: ' ';
  display: inline-block;
  width: 0;
  padding-top: 100%;
  vertical-align: middle;
}