1:1 aspect ratio image

foo.css

div.thum {
    position: relative;
    width: 100%;
    padding-top: 100%; /* initial ratio of 1:1*/
}

div.thum > img {
    width: 100%
    position: absolute;
    top:      0;
    left:     0;
    bottom:   0;
    right:    0;
    width: 100%;
    height: 100%;
    object-fit: cover;
}