title: kivy: how to make an image act like a button
body: <p>I'm new to Kivy. I want an image to change into a different one when it is pressed (touched). 
  There should be some easy way of doing this but I can't figur out what. </p>
  
  <pre><code>&lt;Game&gt;
     Image:
       id: "11"
       center_x: root.width / 2 - root.height * 1/5 * 1.5 
       center_y: root.height * 1/5
       size: root.height / 6, root.height / 6
       source: root.images[0]
  </code></pre>
  
  <p>This is part of my code. I don't know how to make my image 'pressable'. Does it have to be a button of some sort? </p>