Nothing.
However there are quite few GIMP and Photoshop tutorials on the net and some specifically for modding TW games on .org and TWC, I'd advice you to stry them out. The .bmp format seems to suggest you didn't really use either of those programs, but paint or something. While there are decent alternatives one should try to gradually learn to use a program of at least the complexity of The GIMP or Photoshop. If you pt some time in it, you can really make some stuff of much higher quality.
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