I modified a couple of .tga files and saved them. The first time I "saved as" .tga and selected both the options (start from lower left and the other I can't remember right now) and another time I simply saved the original, without any options. Both times I tried to load the game and it failed on me (crashed). Once I removed the modified .tga files, the game worked well as normal.
So my question is, how can I modify the .tga files (with gimp) and have them working in the game?