Unexpected Image Scaling

[ChronosWS]

I am trying to set up a background image for my various menu screens, using a TGA file which is presently the same size as my render window (1024x768.) However, it appears for some reason that this image is being stretched vertically about 30% and I am not sure why.

My imageset:

<?xml version="1.0" ?>
<Imageset Name="MainMenu" Imagefile="MainMenu.tga" NativeHorzRes="1024" NativeVertRes="1024" AutoScaled="false">
<Image Name="Background" XPos="0" YPos="0" Width="1024" Height="768" />
</Imageset>

The relevant window:

<?xml version="1.0" ?>
<GUILayout>
<Window Type="DefaultGUISheet" Name="MainMenu">
<Window Type="TaharezLook/StaticImage" Name="MainMenu/Background">
<Property Name="Position" Value="x:0.0 y:0.0" />
<Property Name="Size" Value="w:1.0 h:1.0" />
<Property Name="RelativeMaxSize" Value="w:1.0 h:1.0" />
<Property Name="BackgroundImage" Value="set:MainMenu image:Background" />
<Property Name="Disabled" Value="True" />
</Window>
...

I have been able to get the aspect ratio to look correct by increasing the width to about 1.33, but then both the right and bottom parts of the image are cut off instead of just the bottom part.

My current theory is that this somehow has something to do with the fact that textures (or internal texture storage) is square. The following is from the runtime dump:

Texture: MainMenu.tga: Loading 1 faces(PF_R8G8B8,1024x768x1) with 0 generated mipmaps from Image. Internal format is PF_X8R8G8B8,1024x1024x1.

NOTE: I have tried various settings for NativeHorzRes and NativeVertRes, as well as turning AutoScaled on and off, but nothing has had the desired effect.

Is there some setting somewhere I need to tweak to make this work, or should I convert my source image to 1024x1024?

Thanks.