Creating ground cover is easier than ever with the Create-A-World Tool. Simply drag and drop your DDS files straight to your Documents >> Electronic Arts >> The Sims 3 Create A World Tool >> UserToolData >> SourceTextures folder and you're done! Then under Terrain Paint tool in the editor, choose your base layer (or add new layer), edit it, choose your new texture and pick your material. Simply press the auto-paint to do complete cover, or whip out your favorite brush to slap it on.
I've created a few fantasy textures to get you started. All my stock image DDS files will be free use policy. I do request link back, but don't require it. I would like to see the finished projects my files were used to create. Just don't claim you created them, or sell them or any derivation of them. This is free, only stock textures were used to create them, and free it shall remain. I'll post up link backs to stock materials used.
You can find all my CaW creations in the Rejected Repository on the Rejected Wiki. Please check out the entire Create-A-World section for tips and tricks, tutorials and walkthroughs, and forum links.