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

.hero-image {
    background-image: url('nejlagiris.png'); 
    background-size: cover;
    background-repeat: no-repeat;
    background-position: center;
    height: 100vh; 
    width: 100%;
}

@media only screen and (max-width: 600px) {
    .hero-image {
    background-image: url('nejlamobil.jpg'); 
}
}