* {
   padding: 0;
   margin: 0;
   box-sizing: border-box; 
}

body {
   background-image: url("desktop.jpeg");
   background-repeat: no-repeat;
   background-size: cover;
}

@media only screen and (max-width: 600px) {
   body {
      background-image: url("mobile.jpeg");
      background-repeat: no-repeat;
      background-size: cover;
   }
 }