body { 
    background-image: url('http://wrsmiths.com/images/slide-bg.jpg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center; 
}

img {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translateX(-50%) translateY(-50%);
    max-width: 100%;
    max-height: 100%;
}