advanced rendering for AutoCAD
It's difficult to figure out how this thing is in 3D. Try to send solid or even wireframe image of it.
If I got it right, you are talking about mapping planar bitmap on vertical triangle surface. I think u have to do the whole rectangular bitmap in photo editor and than use planar mapping option in NXT. For the upper left corner you can use point filter to get local .Y coord of the triangle top corner and az .X coord the left lower corner.
This sort of thing works better with UV mapping, which AutoCAD does not have. Blender has good UV mapping-- you can export an .obj file and import it with nXt. We just had a discussion on the topic here.