For make a good model texture you need gimp, blender, and the model (and the original texture)
In gimp open the texture and save as a other file on the desktop to be easily found
Then in blender open the model
or if it's a md3 use a python script in the text editor of blender
when you have your model you need to have three or four windows:
the 3D view, the text editor to open md3, the buttons window, and the UV image window.
select your model, switch in edit mode. In the UV image window load the texture you have saved on your desktop
In the 3D view switch the appearance of the model from "solid" to "textured"
Now you should see your model with textures.
the final step is to switch from "Object mode" to "Texture paint".
Now you can paint on your model, or the tremulous models
you can change the properties and the colors of the brush in the buttons window
save your texture on the desktop and continue to edit with gimp