I've always loved the style of the original BG1 NPC portraits - they're far and away my favorite of any RPG, including the other IE games. However, I won't pretend that they're entirely perfect, either. So for me, the ideal NPC portrait replacement is one that stays fundamentally
true to the original, but makes a few tweaks/"upgrades."
For example, I don't like that a number of the original portraits are cluttered with sparkle/glow effects that obscure the character itself. So I use this alt portrait for Kagain, which removes those effects so that you can clearly see his face and helmet (the latter of which looks quite nice).
In the same vein, I also use this "cleaned up" portrait for Khalid.
I love this portrait of Kivan, which gives his face a more chiseled, "tough guy" look, more befitting his "Dirty Harry"-esque persona and voice set, with a look of determination/vengeance in his eyes.
This is what Viconia was always meant to look like IMO - her skin tone is more appropriate for a drow than in either of her original BG portraits, and a red gleam in her eyes hints at her evil essence. I love the shimmering armor style too, which reminds you that she is indeed a cleric (unlike her BG2 portrait, which makes her look like a hooded mage).
I've always gotten the impression that Branwen was intended to be attractive - i.e.: the carnival master describes her as a "warrior maiden," and both Coran and Ajantis will openly marvel at her beauty - but her original portrait is rather plain IMO. This portrait is closer to how the game portrays her IMO.
This portrait of Safana trims away the folds of flesh under her chin to give a (more appropriately IMO) younger, leaner look.
In some cases, a bit more "overhauling" is required for portraits that, regardless of their inherent quality, just don't suit the character. This was never more apparent in the case of Coran, whose original portrait portrays him as an ancestor of the Joker rather than a debonair elf. This portrait is much closer to how he's portrayed in the game IMO.
Although I do love Imoen's portrait, like many people, I also agree that it looks too old/mature for her character. This is an ideal alt portrait for me, which gives her a younger, leaner look, while still retaining her distinctive cheekbones and smirk.
I use this portrait for Faldorn, which is better suited to her quiet, enigmatic demeanor than the snarling original IMO.
In one of the rare instances in which I've replaced an NPC's portrait with something entirely different, I use this portrait for Neera. I feel that the short, "punky" hairstyle better captures her spunky personality, and the scar on her face is suggestive of the many travels and hardships that she's had to endure before meeting you (or perhaps, a mishap caused by one of her wild surges).
Thoughts? What are some examples of alt portraits that you folks use?