Jeff Bridgman is correct. All you need is
background: url('pic.jpg')
and this assumes that pic is in the same folder as your html.
Also, Roberto's answer works fine. Tested in Firefox, and IE. Thanks to Raptor for adding formatting that displays full picture fit to screen, and without scrollbars... In a folder f, on the desktop is this html and a picture, pic.jpg, using your userid. Make those substitutions in the below:
<html>
<head>
<style>
body {
background: url('file:///C:/Users/userid/desktop/f/pic.jpg') no-repeat center center fixed;
background-size: cover; /* for IE9+, Safari 4.1+, Chrome 3.0+, Firefox 3.6+ */
-webkit-background-size: cover; /* for Safari 3.0 - 4.0 , Chrome 1.0 - 3.0 */
-moz-background-size: cover; /* optional for Firefox 3.6 */
-o-background-size: cover; /* for Opera 9.5 */
margin: 0; /* to remove the default white margin of body */
padding: 0; /* to remove the default white margin of body */
overflow: hidden;
}
</style>
</head>
<body>
hello
</body>
</html>