Actually there is a much easier way.
In your planets folder you have the BMP folder with all your graphics that is used for all your games.
If you want to have custom graphics for one specific game slot while the rest stay the normal, you have to create a new BMP folder for that game slot. Winplan will then use that BMP folder over the normal one.
It is important that all the required graphics exist in your game slot BMP folder as well, so I suggest you copy the original folder.
This is valid for Winplan original as well as Winplan 32bit.
Your planet folder is at
Your normal graphics folder is at
If you use game slot 2 for your game and want custom graphics only for it
If you use game slot 5 for another game with custom graphics only for it