body {
  height: 100%;  
  background-image: url("../imagenes/fondo_principal.jpg");
  background-size: cover;
}
.main_bg {
    background-image: url("../imagenes/fondo.jpg");
    background-size: cover;
}
.bg {
  background-image: url("../imagenes/fondo.jpg");
  background-position: center;
  background-size: cover;  
}