.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('2015/last4/1.jpg');}
       
.img_2{background-image:url('2015/last4/2.jpg');}
       
.img_3{background-image:url('2015/last4/3.jpg');}
       
.img_4{background-image:url('2015/last4/4.jpg');}
       
.img_5{background-image:url('2015/last4/5.jpg');}
       
.img_6{background-image:url('2015/last4/6.jpg');}
       
.img_7{background-image:url('2015/last4/7.jpg');}
       
.img_8{background-image:url('2015/last4/8.jpg');}
       
.img_9{background-image:url('2015/last4/9.jpg');}
       
.img_10{background-image:url('2015/last4/10.jpg');}
       
.img_11{background-image:url('2015/last4/11.jpg');}
       
.img_12{background-image:url('2015/last4/12.jpg');}
       
.img_13{background-image:url('2015/last4/13.jpg');}
       
.img_14{background-image:url('2015/last4/14.jpg');}
       
.img_15{background-image:url('2015/last4/15.jpg');}
       
.img_16{background-image:url('2015/last4/16.jpg');}
       
.img_17{background-image:url('2015/last4/17.jpg');}
       
.img_18{background-image:url('2015/last4/18.jpg');}
       
.img_19{background-image:url('2015/last4/19.jpg');}
       
.img_20{background-image:url('2015/last4/20.jpg');}