I've seen this trick on many websites and I want to make it for a project of my own. I have an image `background.jpg` that is VERY BIG, just to be ready for any screen size. I wish to center the background image to the visitor's window. I mean, while scrolling the window content will move but the background will stay in place - centered vertically and horizontally. BTW most of the viewers will have old crappy PC's so it'd better not flicker, if it'll be JS.