1) Locate your Tyranny folder:

steam folder/SteamApps/common/tyranny

2)Make a backup folder for your player folder, it's located here:

steam folder/SteamApps/common/tyranny/Data/data/art/gui/portraits/player

3) Unpack respective .zip archive somewhere on your computer.

4) Pick the newly created folder (from .zip archive) and then:

5a)Drag and drop it in the main folder of the game.

5b) Locate player folder in the unpacked archive and drag&drop it directly in the
steam folder/SteamApps/common/tyranny/Data/data/art/gui/portraits/

It won't override anything, unless you have more than default player portraits or i screwed up something.