html, body{height: 100%; width: 100%; margin: 0;}

.cover{ width: 100%; height: 100%;}
img{width: 100%; height: 100%;}

@media (min-width:999px){
        .cover{width: 100%; height: auto;}
        img{width: 100%;}
} 

/*---Tablet---*/
@media (max-width: 998px){
         .cover{max-width: 998px; height: auto;}
         img{max-width:998px ;}
}

/*---Smartphone---*/
@media (max-width: 767px){
        .cover{max-width: 767px; height: auto;}
        img{max-width: 767px;}
}