Are there any mods which add portraits to non-joinable npcs? I tried adding them myself using NearInfinity but I must be doing something wrong as the changes are not visible ingame.

I don't know of any for vanilla BG1, but the project adds loads, as does [url=http://kerzenburg.baldurs-gate.eu/downloads.php?id=69]NSC Portraits. Both are for BGT or TuTU, though.

To add them yourself with NI, you need to assign them to each individual .cre file. A daunting task if you want to do a great many.

I tried adding one just to see if it works but it doesn't. Maybe you cannot add them to vanilla BG?

For example, I opened gorion.cre in NI, added gorions.bmp (which I earlier put in the override folder) in the small portrait field and saved the changes but there is no effect.

It looks that way. If you check the cre file for Imoen in AR2600 (Candlekeep tutorial) it has a non-Imoen portrait -- the same as the mage in the practice room -- assigned to it, but in game no portrait displays. I had never taken much notice before.

If you add in portraits AFTER the creature has been created in-game then it won't show up until a new playthrough.
It seems the creature files get stored in the save-game somewhere (or at least certain states for them).

ALSO: The portrait you want to use must be in the override folder (not the portraits folder - that's only for PC portraits AFAIK). (edit - I know you've done that - this is just general info for anyone reading the thread).
AND it must be the right size and file-type (ie 38x60 .bmp 24-bit)

For joinable npcs there are several creature files - but for those you only need to drop the correctly named (and sized and type) files into the override folder.

Try starting a new game and seeing if your gorion portrait shows up).

@TrollumThinks

I think that unmodified BG1 engine just can't handle it. That's why BG1NPCs project is for tutu/bgt only. Like Hickory said, even characters with set portraits from the beginning don't show their portraits in dialog.

Oh, ok then.
I'm playing with BGT and had portraits not show up because I added them after the creature was created.
Seems a little odd that the creature files in vanilla can't be updated to include a portrait but joinable NPCs can have them show up when joining. Or maybe it's the earlier engine that doesn't display portraits in dialogue mode (?) - do joinable NPCs show their portraits in dialogue box when asking to join?

No, they don't. I just checked (in a new, unmodded game) with Montaron, Xzar, Jaheira and Khalid. No dialogue portraits in vanilla BG1.