Its actually really easy, if you have a program that can read .png images. Just open up any of the race images in the games graphics directory, and paste in any Borg picture you have. Save it in the same directory, making sure to change the number at the end of the filename.
So, if I were to do it I would:
1st: Navigate to the directory called C:\Program Files\Stardock\TotalGaming\GalCiv2\Gfx\Race
2nd: Open up the file called RaceImage59.png in Photoshop or Fireworks
3rd: Paste the Borg picture into the image making sure not to change the image's size
4th: Save the new image as RaceImage60.png in the same directory.
You can do the same thing for logos too. Once you have it down, you'll have craploads of pointless portraits and logos just like I do.
Happy Assimilating!
As an uninteresting sidenote, the cute little Belldandy logo next to my name is the one I use in the game, when pursuing a more pacifistic victory...