title: background image change with screen size
body: <p>My website's background is a huge image that covers the whole webpage, how can I make it so that, we detect user's screen size and change the background image accordingly?</p>
  
  <p>Say for example, my background image is 1x1px, if user screen is 2x2px (this is just an example, nobody has this kind of small screen), I want stretch my background to fit 2x2. If user screen is say, 0.5x0.6px, then I want my background to be shown only its 0.5x0.6px part, not the whole 1x1px.</p>