1. Fisrt back-up your flag folder 
   (Distant Worlds\images\ui\flagshapes)
2. Replace the flag you want by one of my flag by renaming
   one of my flag and then place it in the flagshape folder.
   Overwrite the file (all the original flag are back-up)
  
   Ex: German Eagle -> flag3

   I have included pictures of the original flag set 
   and my flag sets so you could easily choose the flags. 

   Some flag work better than other.