Just go to your /My Games/GalCiv2/gfx/portraits folder and copy the image to there. It is important, that it is in *.png format, you need a graphics software that can handle it and save images as png. (I use Macromedia Fireworks, but for a really good freeware graphics software check www.gimp.com) I do not know for sure but it may be, that you need to adjust the size to the exact value needed. For race pics this is 256x256 pixels. I do not know if they get resized if you have another size. If you want to changel logos etc as well just look for the standard pics in your game directory to get the correct size needed.