responsive.css

#img01 .img {
    background-image: url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_900.jpg);
    padding-bottom: 70.4444444444%;
    width: 100%;
}

@media(max-width: 542px) {
    #img01 .img {
        background-image:url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_600.jpg);
        padding-bottom: 70.3333333333%
    }
}

@media(min-width: 542px) and (max-width:692px) {
    #img01 .img {
        background-image:url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_900.jpg);
        padding-bottom: 70.4444444444%
    }
}

@media(min-width: 692px) and (max-width:992px) {
    #img01 .img {
        background-image:url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_600.jpg);
        padding-bottom: 70.5%
    }
}

@media(min-width: 992px) and (max-width:1292px) {
    #img01 .img {
        background-image:url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_900.jpg);
        padding-bottom: 70.4444444444%
    }
}

@media(min-width: 1292px) and (max-width:1592px) {
    #img01 .img {
        background-image:url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_1200.jpg);
        padding-bottom: 70.5%
    }
}

@media(min-width: 1592px) {
    #img01 .img {
        background-image:url(http://cdn.theatlantic.com/assets/media/img/photo/2015/01/photos-of-the-week-13-19/w01_RTR4K9Z8/main_1500.jpg);
        padding-bottom: 70.4666666667%
    }
}