Here is the neat little block of CSS that I use to resize a background image to fit the size of the browser window the site is being viewed in:
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;
}Let me know if you have any questions or need help with this! 