Take a copy of the image you like and save it somewhere on your computer.
Go into your Triple A. You can either change it for one game or you can change it for all the games.
If you want to change it for one particlar game you can replace the image in the “units” folder inside the Map folder.
If you want to change the image for all games then you can go into the “assets” folder in triple A and go into the “Units” folder.
It all depends on whether the game you are playing has its own “unit” folder or not. If a game has its own unit folder then the unit images have to be changed in that. If the game has no units folder it will draw images from the Triple A unit folder.
The changing of unit images is also dependent on how the games you have, have been installed. If all your games are still zipped then there may be other steps involved.