.đầu {
    background-image: url(https://upload.wikimedia.org/wikipedia/commons/thumb/5/57/NASA-Apollo8-Dec24-Earthrise-b.jpg/640px-NASA-Apollo8-Dec24-Earthrise-b.jpg);
    background-size: cover;
    width: 100%;
    padding-top: 100%; 
    position: relative; 
}