.parent{

  width:100px;

  position: relative;

  border: 1px solid black;

  height: 50px;

}

.inner {

  padding-top:100%;

  position: relative;

  width:100%;height:100%;

}

​

img{

  width:100%;position:absolute; top: 0;right:0; bottom:0;left:0; margin:auto;

}