#key - Centrer une image verticalement et horizontalement
Le code :
 
#container {
    max-height: 100%;
}

#container img {
    /* Keeps image from going outside the screen */
    max-height: 100%;
    max-width: 100%;
    /* Keeps image from beeing distorted */
    height: auto;
    width: auto;
    /* centering horizontally AND vertically */
    /* 50% of container */
    position: relative;
    top: 50%;
    left: 50%;
    /* 50% of image */
    transform: translate(-50%,-50%);
}