While the source textures for Walls are created in a 1x2 aspect ratio, the game displays it in a 1x3 aspect ratio. To get the images on your custom content Walls to appear unstretched in the game, you can follow these steps:
1) Take the .BMP from the Working Directory and resize it to 256 pixels wide by 768 pixels tall
2) Create your custom Wall pattern
3) Resize the image to 256x512 pixels
4) Save the .BMP and click refresh in HomeCrafter Plus
Question and answer provided by the
official FAQ.