.ow_header_pic{background-image:url(https://miss-lisa.fr/ow_userfiles/themes/theme_image_31.jpg)}
.ow_logo_img{background-image:url(http://miss-lisa.fr/ow_userfiles/themes/theme_image_18.png)}
<style>

@media only screen and (max-device-width: 480px) {

    .ow_sidebar{

        clear:both;

    }

}

</style>