css for setting image background for all size screen code example
Example 1: how to cover full image in css
body {
background-position: center;
background-repeat: no-repeat;
background-size: cover;
}
Example 2: css background image size fit screen
html {
background: url(images/bg.jpg) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}