Vista - 'Default' source for an Image

Hi,
I'm trying to display a default image in an Image control when the
Source property is null.

Unfortunately, a DataTrigger which checks for {x:Null} in the Source
property, and sets another value to Source in the trigger does not
work, as the trigger property and can not be repeated in the trigger
setters:

<Style x:Key="defImageStyle" TargetType="{x:Type Image}">
<Style.Triggers>
<Trigger Property="Source" Value="{x:Null}">
<Setter Property="Source" Value="defaultPicture.jpg"/>
</Trigger>
</Style.Triggers>
</Style>

Is there a way to achieve this in Xaml?

Thanks,
Luc

How about overriding the Template property such that it contains a hidden
default image that's visibilty gets flipped based on the Source trigger?