Hallo Feuer Frei,
Blender and 3dWings are programs to add or change 3d objects. If you want to add a cube behind your wintergarten, this is what you need. I guess they don't even open TGA files.
(TGA is a format like JPG or GIF - for pictures/images -
http://en.wikipedia.org/wiki/.tga)
CS2 or Gimp or Paint.net are the tools to change camouflage or do paintjobs or change any "flat 2d" screens for the game. They do open TGA files.
For the camouflage job you have to find the "skin" (TGA) wrapped around an ingame 3d object and change it to your liking. The ingame screens are TGA files too.
There are thousands of tutorials on the internet for both groups of tasks - especially for the painting stuff.
For i.e. Gimp:
http://www.gimp.org/tutorials/