When The Sims 2 places Ground Cover textures on a Lot, the texture is actually mirrored by the game. In order to show you how your custom content is going to appear in the game, we do the same in the HomeCrafter Plus preview window.
Most graphics programs will allow you to "flip" or "invert" your image so that when you import your custom content, it will appear as you want it in the game. Note that if you do flip your image, the thumbnail will now appear reversed.
Question and answer provided by the official FAQ