title: Keep two images relatively scaled when window resized
body: <p>Assume 2 images, a big one for Background and an smaller one for Foreground.  The foreground stays exactly at the middle of the background, covering a relative amount of it (e.g. 50%) so the User sees them as a single image.</p>
  
  <p>The problem is that I need the Background to fill the <code>Window</code> in various sizes. Setting its <code>Stretch</code> value to <code>UniformToFill</code> will do that. But how can I keep the foreground still covering 50% of the background?</p>