Adding Text to Textblock in StackPanel does not move Image, Silverlight

Question

I have the following xaml:

 <StackPanel Orientation="Horizontal" x:Name="HeadlineSP">
         <TextBlock x:Name="CanvasHeadline" Text="Sample" Margin="8,8,0,0" />
         <Image Source="http://localhost/myimage.png" />
 </StackPanel>

when i update the text of CanvasHeadline the Image does not move right to left dependiong on the width of the text.

CanvasHeadline.Text = "longstringaaaaaa";

or

CanvasHeadline.Text = "short";

and the image stays in the same location, where I would expect it to move and stay at the end of the text.