I don't see a folder for the game under AppData/Roaming under any combination of Pathfinder, Kingmaker, GOG or Owlcat. I see posts on the internet saying that there is supposed to be a Custom Portraits button in the character creation screen. There is not.
Edit: Figured out the problem. The custom portrait button is unlabeled. It is a little abstract head-and-shoulders icon beneath the portrait array. Also, the folder is apparently in AppData/LocalLow.
Also: Apparently, each custom image gets its own folder. You press the Open Portraits Folder button and replace or edit the ones named Fulllength.png, Medium.png and Small.png.
The formats are as published elsewhere:
Small.png - width: 185px, height: 242px;
Medium.png - width: 330px, height: 432px;
Fulllength.png - width: 692px, height: 1024px;
Post edited September 26, 2018 by JohnnyAngel