title: How to set a default source for an Image if binding source is null?
body: <p>I'm using binding for source of an <code>Image</code> control.</p>
  
  <pre><code>&lt;Image Source="{Binding ImageUri}"/&gt;
  </code></pre>
  
  <p>But this <code>ImageUri</code> can be null, therefor I want to use a default image, a place holder, for that, which can be in <code>/Assets/PlaceHolder.png</code> for example.</p>
  
  <p>How can I set a default image? thanks.
  <em>(It's a WP8 app, but should not be different of WPF)</em></p>