title: Flutter how to change image after onTap with Gesture Detector
body: <p>I have a Image inside a GestureDetector widget. I want to change this image when <strong>onTapDown</strong> is called, and then change it again when <strong>onTapUp</strong> is called. Is there possible to do that? 
  In other app (Native Android app with java), I used to do it using a button and changing its background with a selector xml, like this:</p>
  
  <pre><code>&lt;selector xmlns:android="http://schemas.android.com/apk/res/android"&gt;
  &lt;item android:state_focused="true" android:state_pressed="false" android:drawable="@drawable/image1" /&gt;
  &lt;item android:state_focused="true" android:state_pressed="true" android:drawable="@drawable/image2" /&gt;
  &lt;item android:state_focused="false" android:state_pressed="true" android:drawable="@drawable/image2"/&gt;
  &lt;item android:drawable="@drawable/image1" /&gt;
  </code></pre>
  
  <p></p>
  
  <p>so, is there a way to do the same in flutter?</p>
  
  <p>---JUST TO BE CLEAR---</p>
  
  <p>In the same widget, I want an image if it is not pressed and a different image if it is pressed.</p>