it depends on which game your using, for sins and entrenchment its a bit diffrent. although generally speaking the same just got more steps in entrenchment.

generally speaking there in the base sins "Texture" window unless your using entrenchment and adding more, in which case you coppy the Player_portrate_(small, large, window or button).tga file over to entrenchment texture folder and modify it adding in the new portrates. Also you got to modify the Player(race).playerThemes and Player(race).playerpictures to let the game know where on those files the new themes are. dont forget to edit the the PlayerThemes.manifest and the PlayerPictures.manifest if your using entrenchment.  Player(race).playerThemes and Player(race).playerpictures are found in the window folder.

maybe, depending on if no bugs are caught. but there still is a few ones in the game that crash it so i doubt it will. then again, we might get lucky and have all bugs fixed with the release.