.bg-img{
    min-height: 100vh;
    width: 100vw;
    object-fit: cover;
    position:absolute;
    background: url(oscar-keys-ojVMh1QTVGY-unsplash.jpg) fixed;
    top:0;
    left: 0;
    z-index: -1;
}

@media (max-width: 768px){
    .bg-img{
        min-height: 30vh;
    }
}