.square {
    float:left;
    position: relative;
    width: 100px;
    padding-bottom : 100px; /* = width for a 1:1 aspect ratio */
    margin:1.66%;
    background-position:center center;
    background-repeat:no-repeat;
    background-size:cover; /* you change this to "contain" if you don't want the images to be cropped */
}

       
.img_1{background-image:url('2006/tokyo2006/1.jpg');}
       
.img_2{background-image:url('2006/tokyo2006/2.jpg');}
       
.img_3{background-image:url('2006/tokyo2006/3.jpg');}
       
.img_4{background-image:url('2006/tokyo2006/4.jpg');}
       
.img_5{background-image:url('2006/tokyo2006/5.jpg');}
       
.img_6{background-image:url('2006/tokyo2006/6.jpg');}
       
.img_7{background-image:url('2006/tokyo2006/7.jpg');}
       
.img_8{background-image:url('2006/tokyo2006/8.jpg');}
       
.img_9{background-image:url('2006/tokyo2006/9.jpg');}
       
.img_10{background-image:url('2006/tokyo2006/10.jpg');}
       
.img_11{background-image:url('2006/tokyo2006/11.jpg');}
       
.img_12{background-image:url('2006/tokyo2006/12.jpg');}
       
.img_13{background-image:url('2006/tokyo2006/13.jpg');}
       
.img_14{background-image:url('2006/tokyo2006/14.jpg');}
       
.img_15{background-image:url('2006/tokyo2006/15.jpg');}
       
.img_16{background-image:url('2006/tokyo2006/16.jpg');}
       
.img_17{background-image:url('2006/tokyo2006/17.jpg');}
       
.img_18{background-image:url('2006/tokyo2006/18.jpg');}
       
.img_19{background-image:url('2006/tokyo2006/19.jpg');}
       
.img_20{background-image:url('2006/tokyo2006/20.jpg');}
       
.img_21{background-image:url('2006/tokyo2006/21.jpg');}
       
.img_22{background-image:url('2006/tokyo2006/22.jpg');}
       
.img_23{background-image:url('2006/tokyo2006/23.jpg');}
       
.img_24{background-image:url('2006/tokyo2006/24.jpg');}