I'm not very confident that I can fix that particular situation. I'll give it some thought though.
When looking straight ahead, the image is a square with no transparency needed. The side view for walls are still square but use transparency above and below the desired wall pixels. The backdrops don't have that. Some way of copying the odd shape is needed (rhombus?), but DirectX only copies rectangular areas. If I copied the backdrop using the same rectangle as the side view wall, you would see the backdrop above and below the wall too.
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CocoaSpud
Dungeon Craft Development Team
http://uaf.sourceforge.net