You can change your Main Character's portrait during the game or in Character Creation.
Here is a sample of custom portrait:
To make your own portrait, you need to create 2 different images. A larger one of 210x330 pixels, a small one of 76x96 pixels, both in PNG format. You can use Photoshop to resize your images and convert format.
Name the larger image xxx_lg.png (xxx is the name you want it to be), and small one xxx_sm.png.
Put these 2 images in your game folder, for example: SteamLibrary/steamapps/common/Tyranny/Tyranny_Data/data/art/gui/portraits/player/male (or female, depends on your character's gender)
Now you can chose your customized Portrait in Character Creation, or switch during game.
Change Portrait of Compainions
Theoretical, you can change the Portrait of your Compainions as well. Simply create a large image (210x330 pixels in PNG format) and a small image (76x96 pixels in PNG )， and replaces the images in \SteamLibrary\steamapps\common\Tyranny\Tyranny_Data\data\art\gui\portraits\companion folder. Be careful, you may want to create a buckup before you make any changes.
A sample of custom Compainion portrait.