body {
background-image: url("img_tree.gif");
background-repeat: no-repeat;
background-attachment: fixed;
background-position: top center;
background-size: contain;
}
html {
background: url(image.jpg) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}