body {
    background-image: url(https://i.ibb.co/Z1knpSt/bg-jpg-1.jpg);
    background-repeat: no-repeat;
    background-position: center;
    background-attachment: fixed;
    background-size: 500px;
    z-index: -1;
}

@media (max-width: 768px) {
    body {
        background-image: url(https://i.ibb.co/Z1knpSt/bg-jpg-1.jpg);
        background-repeat: no-repeat;
        background-position: center;
        background-attachment: fixed;
        background-size: 350px;
        z-index: -1;
    }
}