html, body, #container {
    height: 100%;
}

#container {
    width: 100%;
    position: relative;
}

#container > img {
    width: 100%;
    max-width: 512px; /* real image width */
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    margin: auto;
}