.square {
    float:left;
    position: relative;
    width: 19%;
    padding-bottom : 0%; /* = width for a 1:1 aspect ratio */
    margin:1%;
    background-position:center center;
    background-repeat:no-repeat;
    background-size:contain; /* you change this to "contain" if you don't want the images to be cropped */
}