.hero .hero-media {
  /* Été */
  /*background-image:url(//s3.amazonaws.com/imagescloud/images/rf/camp_mercier/20150408-m.jpg);*/
  /* Hiver */
  background-image:url(//s3.amazonaws.com/imagescloud/images/rf/camp_mercier/20191021-m.jpg);
}

@media only screen and (min-width: 768px) {
  .hero .hero-media {
    /* Été */
     /*background-image:url(//s3.amazonaws.com/imagescloud/images/rf/camp_mercier/20150408.jpg);*/
  /* Hiver */
    background-image:url(//s3.amazonaws.com/imagescloud/images/rf/camp_mercier/20191021.jpg);
  }
}