Resize image from url for panorama background

Question

I'm working on my first WP7 app and now I'm exploring the world of the panorama control. I've loaded an object from a REST service and I've set the background of the panorama to the image url in that object.

var brush = new ImageBrush() { ImageSource = new BitmapImage(new Uri(_group.photo_url, UriKind.Absolute)) };
groupPanorama.Background = brush;

The problem I have is that the image is an arbitrary size on the server and this case ends up being squished (horizontally) and tiled without even filling the width of the screen horizontally.

I'm assuming I need to re-size this to dimensions that the control supports, but I am not sure how to go about this. I assume this is a pretty common task.

many thanks!

EDIT:

Including a snip-it of the xaml. Pretty generic:

<!--LayoutRoot contains the root grid where all other page content is placed-->
<Grid x:Name="LayoutRoot">
    <controls:Panorama Name="groupPanorama" Title="" SelectionChanged="groupPanorama_SelectionChanged">

        <!--Panorama item one-->
        <controls:PanoramaItem Header="Main">
            <Grid/>
        </controls:PanoramaItem>

I guess I should note that I have tried the various Stretch properties for the ImageBrush already with no luck.

UPDATE:
This appears to work with normal size images. The image I was originally attempting to do this with is pretty small and doesn't appear to stretch. Perhaps it's a bug, but I'll work around this for now.