One of the quickest ways to give style to a webpage is to use a full screen image as its background. It can be done with a surprising little bit of CSS. This code will fill the entire background with a centered and scaled image that doesn't cause scrolling and works in all major browsers.

html { 
  background: url("http://i.imgur.com/uO3puXn.jpg") no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}