how to make fixed background image in css

1body {
2  background-image: url("img_tree.gif");
3  background-repeat: no-repeat;
4  background-attachment: fixed;
5}
6

1body {
2  background-image: url("img_tree.gif");
3  
4  background-position: center;
5  background-repeat: no-repeat;
6  background-attachment: fixed;
7  
8  background-size: cover;
9}

1body {
2  background-position: center;
3  background-repeat: no-repeat;
4  background-size: cover;
5}

1fixed background