People use Paint Programs - do a search for skining, lots of good information here.
When you are in the TE (terrain editor) and in the Object mode, look for a plane ( in the available shapes box ) that you want to skin.
Highlite it and then click on the Save Texture button.
The TE will make a subfolder in your terrain name folder called savedtex.
It will show all the .bmp that go with that plane.
Pokie