background size in css

1#example1 {
2  background: url(mountain.jpg);
3  background-repeat: no-repeat;
4  background-size: auto;
5}
6
7#example2 {
8  background: url(mountain.jpg);
9  background-repeat: no-repeat;
10  background-size: 300px 100px;
11}

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

1body {
2
3    background-image: url("img_tree.gif");
4
5   background-repeat: no-repeat;   
6 background-attachment: fixed;
7  
8   background-position: top center;
9  background-size: contain;
10
11 }

1#example1 {		/* Example1 has an auto background size */
2  background: url(mountain.jpg);
3  background-repeat: no-repeat;
4  background-size: auto;
5}
6
7#example2 {		/* Example2 has an fixed background size */
8  background: url(mountain.jpg);
9  background-repeat: no-repeat;
10  background-size: 300px 100px;
11}

1Background Size is Responsible for Setting a Background picutre in a given
2Contaiener in which you apply that special Background
3
4/* This will make width as static and height as auto */
5/* These three are quite same */ 
6background-size: 100vw auto;
7background-size: 100%;
8background-size: cover;
9
10
11/* for getting full width and height every time */ 
12background-size: 100vw 100vh;