Changing NPC portraits is a little different. NPC's use three portrait files and there is no in-game way to change them.

The correct naming pattern and sizes for the portrait files are as follows:

xxxxS.bmp : 38 x 60 px
xxxxM.bmp : 110 x 170 px
xxxxL.bmp : 210 x 330 px

Make sure the images are saved as 24-bit BMP format.

Use this list to check the correct file names for each NPC:
https://baldursgate.fandom.com/wiki/Portrait_File_Names

You then need to place the files in the game's "Override" folder, which should be a sub-folder of the game's main folder. For example if you installed BG1 to C:\GOG Games\Baldur's Gate then your override folder would be C:\GOG Games\Baldur's Gate\Override. If that folder doesn't exist, create it.

Ryan333,

Thank you so much for the very detailed explanations. Great, now it works!

I played the old edition when it was published in 1998. Waiting for Baldur's Gate 3 pre-release, yesterday, after 22 years I installed the Enhanced Editions of Baldur's Gate and Baldur's Gate II. Cool to play an Enhanced Edition with Enhanced portraits.

Awesome! Glad you got it working. Good fortune in your travels!

Thanks!