.thumbnail_container {
     position: relative;
     width: 100%;
     padding-bottom: 100%;
     margin-bottom:10px;
}

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