.main-content.is-fullscreen:before {
  /* Été et Automne */
   background-image:url("//s3.amazonaws.com/imagescloud/images/abonnement/courriels/20151021-m.jpg"); 
  /* Hiver */
   /*background-image:url("//s3.amazonaws.com/imagescloud/images/abonnement/courriels/20151022-m.jpg");*/
}

@media only screen and (min-width: 768px) {
  .main-content {
    /* Été et Automne */
     background-image:url("//s3.amazonaws.com/imagescloud/images/abonnement/courriels/20151021.jpg"); 
    /* Hiver */
     /*background-image:url("//s3.amazonaws.com/imagescloud/images/abonnement/courriels/20151022.jpg"); */
  }
}