Created Unassigned: Zoombox: How can I get my Image at the center of ZoomBox [22263]

Hi:
I want to the Image named FilmImage is presented at center, but it is still at the topleft corner stubbornly.
What should I do to it right?
XAML Code is:
<Grid>
<Border BorderThickness="13" BorderBrush="Red">
<xctk:Zoombox IsUsingScrollBars="True" VerticalContentAlignment="Center" HorizontalContentAlignment="Center">
<Image x:Name="FilmImage"
Stretch="None"
Source="Images/894.jpg"
HorizontalAlignment="Center"
VerticalAlignment="Center"/>
</xctk:Zoombox>
</Border>
</Grid>

thanks a lot.