Sorry if my meaning was not clear - this is not possible, at least in the classical sense. Modifying files (as in a PHP vs CSS argument) is not what I was referring to.
Background stretching is only supported in CSS3 by the
background-size attribute and AFAIK the latest version of Opera is the only
browser that offers some support for it (and that too I am not sure). You can "fake" it by following the instructions here:
http://webdesign.about.com/od/css3/f/blfaqbgsize.htm.