When exporting TGA files directly from GIMP or other tools, make sure that
RLE-compression (run-length encoding) is DISABLED.

RLE-compressed TGA files are not loadable by the game code.


If you're having trouble with this, you can always export as PNG and
then convert with ImageMagick like so:

convert ground_6.png -auto-orient -type truecolormatte ground_6.tga