GIMP is an opensource alternative to applications such as Photoshop. You can get it here: Official GIMP Website
The 2 is just the version. It's currently at 2.6 if I remember it correctly.
We shall not cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.
^What he said. You can basically use any image editing or creating software to make your texture, I just used GIMP as an example. It's the best free software, as far as I know.
The first post says it has to be targa, so it has to be targa. Otherwise it would say that it can be either targa or png. It's really extremely obvious if you just use your brain a LITTLE bit.