Usage:
Somewhere in your script, add:

img.bg-sf {width: 100%; height: 100%; left: 0px; top: 0px; position: fixed; z-index: -1;}
/* 
of course, with this effect, the image will be skewed on some screens.
what i do is make it fade to black on a side, then set the pages background to black and remove either width, or height. you will find it quite easy and useful.
*/