body, html {
    margin: 0;
    padding: 0;
    height: 100%;
}
.fullscreen-img {
    width: 100%;
    height: 100%;
    background-position: center;
    background-repeat: no-repeat;
    background-size: contain;
    margin: 0 auto;
}
/* 电脑端（屏幕宽度大于等于768px） */
@media (min-width: 768px) {
    .fullscreen-img {
        background-image: url("desktop-image.jpg");
    }
}
/* 手机端（屏幕宽度小于768px） */
@media (max-width: 767px) {
    .fullscreen-img {
        background-image: url("mobile-image.jpg");
    }
}