Ground surface textures must be made of 64x64 pixel bitmaps with 24-bit color depth.
Must tile seamlessly horizontally and vertically for proper surface integration.
Use multiple ground texture files for one ground type. These .bmp should fit to each other on the borders but should be a bit different inside.
To create ground surface texture .bmp try using gimpand the resynthesizer plugin. Public domain base pictures to create ground textures can be found here burningwell.org (gimp and resynthesizer are directly available in most linux distributions )