Hello all, it's my first time playing Baldur's Gate and it's finally starting to click with me and I'm quite enjoying it. I bought the series when it was on sale some time ago before the Enhanced Edition was out. As such, I followed this guide posted by GOG:

https-embed.gog.com/forum/general/enhance_the_gameplay_in_your_edition_of_baldurs_gate_from_gogcom

NOTE: I had to edit the link since I'm a first time poster, I guess.

...to "enhance" my game - I now believe I'm running Baldur's Gate I on Baldur's Gate II's engine; essentially I've combined the games, from my understanding.

Anyways, I mention that because it might help in solving my problem, and I don't know how relevant it is to BG modding, although I assume it is. I've gotten to the Friendly Arm in and I've just met Jaheira. Now, for my character, I unknowingly chose her portrait, forcing her to use a backup portrait which not only doesn't fit, but looks different in the record screen and displays a male fighter portrait (?), I believe.

Now, all I want to do is change Jaheira's portrait to her BGII variant, since they look sufficiently different from one another, and have my character continue using the BGI variant. Is this possible at all? Any help would be very much appreciated.

Oh, yeah, that worked like a charm. The only thing is a downloaded a custom potrait pack and found one that I quite liked and also prompted me to change my PC sprite to look more like it, which had the added benefit of further helping me distinguish Jaheira from my player.

Thank you for the help, though; very much appreciated. I assume the same process can be applied to switch Jaheira's potrait to the BGI variant when I get to BGII (since I honestly like that portrait better, and she might as well use it if I'm not going to anymore)? If that's even necessary, that is. I am using BGT.

Not quite. The BG1 portrait is replaced in BG2, so you will need to extract the BG1 bitmaps and add them to your override folder. Though, thinking about it, the files should already be there in BGT. But in principal it's the same. Just remember this: to physically change main game portrait (Jaheira, Imoen, Minsc etc.) images they need to go in the 'override' folder. To add other NPC or character portraits, put them in the 'portraits' folder.

Main character portraits are very specifically named. All portraits are named in the old DOS 8.3 convention also. For example, in Jaheira's case her portraits are named:

BG1
JAHEIRAL.BMP (large)
JAHEIRAM.BMP (small)

BG2
NJAHEIRM.BMP (large)
NJAHEIRS.BMP (small)

Ah, alright, I'll have to remember that when I get to BGII, then. Thank you for all of your help!