.img-preview-size{
    width:15%;
    margin:2px;
}

.img_gallery{
    display: block;
    width: 100%;
    position: relative;
    height: 0;
    padding: 100% 0 0 0;
}

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