I have yet to find anything on this in any WB documentation, but after checking in Skin Studio, I found that if one wants to include the wall as part of a WB skin, the wall must be a bitmap. I tried to load a .jpg anyway, and Skin Studio gave me an error. I then edited the file in notepad, and saved it, but when I reloaded the blind, the wall did not load. So apparently, bitmaps are required if one wants the wall to be included in the WB skin.