title: Passing image object as a button background in Kivy
body: <p>In Kivy, is there a way to pass image object as a button background, instead of image file name?</p>
  
  <p><code>button.background_normal</code> property accepts only strings. I would like to customize image properties, such as <code>allow_stretch = False</code>.</p>
  
  <p>If that succeeds, how can I specify image alignment inside a button, eg. to make it top-left aligned?</p>