how to set background automatically with my screen height


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;
}

body {

  background-image: url("img_tree.gif");

  background-repeat: no-repeat;  
background-attachment: fixed;

background-position: top center;
background-size: contain;

}

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