Simply drop the three images into one of the folders generated by the game when you select the custom portrait option at character creation and use the Visual Adjustments mod to change the companion portrait in-game.

Your portraits folder can be found at C:\Users\Username\AppData\LocalLow\Owlcat Games\Pathfinder Kingmaker\Portraits

The images have been sized and scaled to comply with Pathfinder:Kingmaker's requirements.

Just drag, drop and enjoy!