Oberkommando des Heeres, OKH, Commander in Chief and Chief of Staff flags.
Red army, Hammer and Sickle and unofficial.
Pics used from the wikipedia, in the public domain, credits to the respective authors, Fornax (german) and Thespoondragon and Alkari (Soviet).
To use them:
Download and extract the zip file.
Right click on the desktop icon of the game.
Properties, change icon.
Browse to the folder where you extracted the icons.
Select, open, OK, apply, OK.
Enjoy [:)]
Edit: added a variant of the red star, hammer and sickle.