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

body {
  background-image: url('../images/desktop-screen.png'); 
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
}
@media screen and (max-width:900px) {
    body{
         background-image: url('../images/ipad-screen.png'); 
    }
}
@media screen and (max-width:650px) {
    body{
         background-image: url('../images/mobile-screen.png'); 
    }
}