.landscape {
    position: fixed;
    top: 0;
    bottom: 0;

    width: 100%;
    height: 100%;

    background: #000 url(images/chameleon2.jpg) 50% 100% no-repeat;
    color: #ffffff;
}

.portrait {
    width: 100%;
    height: 100%;
    background: url(images/chameleon.jpg) 50% 100% no-repeat;
    -moz-transition: opacity 1s ease;  /* FF3.7+ */
    -o-transition: opacity 1s ease;  /* Opera 10.5 */
    -webkit-transition: opacity 1s ease;  /* Saf3.2+, Chrome */
    transition: opacity 1s ease;
}