title: datatrigger on enum to change image
body: <p>I've got a button with a fixed background image and would like to show a small overlay image on top of it. Which overlay image to chose depends on a dependency property (<code>LapCounterPingStatus</code>) of the according viewmodel.</p>
  
  <p>This is what I got so far:</p>
  
  <pre><code>&lt;Button&gt;
      &lt;Grid&gt;
          &lt;Image Stretch="None"&gt; &lt;!-- Background Image --&gt;
              &lt;Image.Style&gt;
                  &lt;Style TargetType="{x:Type Image}"&gt;
                      &lt;Setter Property="Source" Value="/Images/Pingn.png"/&gt;
                  &lt;/Style&gt;
              &lt;/Image.Style&gt;
          &lt;/Image&gt;
          &lt;Image Stretch="None" Panel.ZIndex="1"&gt; &lt;!-- Small Overlay Image --&gt;
              &lt;Image.Style&gt;
                  &lt;Style TargetType="{x:Type Image}"&gt;
                      &lt;Style.Triggers&gt;
                          &lt;DataTrigger Binding="{Binding Path=LapCounterPingStatus}" Value="PingStatus.PING_UNKNOWN"&gt;
                              &lt;Setter Property="Source" Value="/Images/RefreshOverlayn.png"/&gt;
                          &lt;/DataTrigger&gt;
                          &lt;DataTrigger Binding="{Binding Path=LapCounterPingStatus}" Value="PingStatus.PING_FAILURE"&gt;
                              &lt;Setter Property="Source" Value="/Images/ErrorOverlayn.png"/&gt;
                          &lt;/DataTrigger&gt;
                          &lt;DataTrigger Binding="{Binding Path=LapCounterPingStatus}" Value="PingStatus.PING_SUCCESS"&gt;
                              &lt;Setter Property="Source" Value="/Images/CheckmarkOverlayn.png"/&gt;
                          &lt;/DataTrigger&gt;
                      &lt;/Style.Triggers&gt;
                  &lt;/Style&gt;
              &lt;/Image.Style&gt;
          &lt;/Image&gt;
      &lt;/Grid&gt;
  &lt;/Button&gt;
  </code></pre>
  
  <p>Relevant parts of my viewmodel</p>
  
  <pre><code>public class ConfigurationViewModel
  {
      public enum PingStatus { PING_UNKNOWN, PING_SUCCESS, PING_FAILURE };
  
      public PingStatus LapCounterPingStatus
      {
          get { return _lapCounterPingStatus; }
          set
          {
              _lapCounterPingStatus = value;
              RaisePropertyChanged(LapCounterPingStatusPropertyName);
          }
      }
  }
  </code></pre>
  
  <p><strong>Right now, no overlay image at all is displayed. What could be wrong?</strong></p>
  
  <hr>
  
  <p><strong>UPDATE</strong></p>
  
  <p>Trace window of my IDE is showing <code>System.ArgumentException</code> and <code>System.FormatException</code>. 
  Could the problem source be a unknown type of enumeration <code>PingStatus</code> im the XAML?</p>