For the background to be attached to the opacity slider, it has to have a specific XML path.
PHP Code:
<Page Name="YourPage">
<ImageStyle Name="SomeImageStyle">
<ImageFrame Source="whatever.dds" SourceRect="1,2,3,4" />
</ImageStyle>
<RectangleStyle Name="SomeRectangleStyle" Center="SomeImageStyle" CenterStretchH="true" CenterStretchV="true" CenterShrinkH="true" CenterShrinkV="true" />
<!-- ... -->
<Page Name="WindowFrame">
<Page Name="Bkg" RStyleDefault="SomeRectangleStyle" />
</Page>
</Page>
You can have the Bkg page point to a RectangleStyle, which points to an ImageStyle which points to an ImageFrame(your DDS). Make sure to make the WindowFrame and Bkg pages the size of your window(more or less).