.bg-header {
    background-image: url(img/header__bg--phone.jpg);
}

@media (min-width: 640px){
  .bg-header {
      background-image: url(img/header__bg--tablet.jpg);
  }
}

@media (min-width: 1024px){
  .bg-header {
      background-image: url(img/header__bg--desktop.jpg);
  }
}