*{padding: 0; margin: 0;}
.arkaplan{
  background: url('nippon-yapim-asamasinda.jpg') no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}


@media only screen and (max-width: 799px) {
  .arkaplan{
    background: url('nippon-yapim-asamasinda.jpg') no-repeat center center fixed;
    background-size: 100%;
    height: 100vh;
  }

}