Stretching a background image with CSS
This simple snippet of CSS will stretch or shrink a background image based on the resolution of the browser.
Add the following CSS to your stylesheet(for WordPress add this under your themes stylesheet):
body {
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;
}