If an NPC's portrait is already taken by the main character, that NPC is given an alternate portrait. For those curious, I made a graphic that shows all of BG1EE's alternate portraits. :
Steam Community :: Guide :: Custom Portraits Guide
Super Ultimate Portrait Pack - BG:EE (ZIP) addon - Baldur's Gate: Enhanced Edition - Mod DB
65 BGEE Portraits ideas | character portraits, fantasy characters, fantasy portraits
Cane's RPG Portrait Pack - 101 top quality fantasy portraits : r/baldursgate
Realistic NPC Portraits for BG2EE at Baldur's Gate 2 Enhanced Edition Nexus - Mods and community
Isandir - Custom Portraits for Baldur's Gate
Isandir (BGII Style) - Isandir
Custom PC & NPC Portraits — Beamdog Forums
Portraits for your Game - Will Update w/date when more added! - 1/13/2020 UPDATE — Beamdog Forums
A portrait pack I drew for BG2EE : r/baldursgate
Steam Community :: Guide :: Custom Portraits Guide
The Deth Guild - Games. Politics. Armageddon. The Fun Stuff.
Isandir - Custom Portraits for Baldur's Gate | Portrait, Character portraits, Baldur's gate portraits