.img-main {
    background-color: #394859;
    background-image: url('images/img-main.jpg');
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center;
}

@media (max-width: 770px) {
    .img-main {
        background-image:  url(images/img-main.jpg);
    }
}