I have the portrait I want, properly sized. I just don't know where to stash it. Data? Scripts?

In the Portrait folder. If there isn't one, you can make one by going into the BG2: SoA directory, right-clicking and select New>Folder. Name it Portraits and stick your pciture in there.

There should be an empty portraits folder in the BG2 main folder. Place them there. It's BG1 where you have to make your own folder because they don't give you one.

Thanks, figured it out now!