If you're saying you have a set of portrait files , all you have to do is extract them into the portraits subfolder in your NWN folder (If one doesn't exist, make one.)
If you have like a picture in jpg format that you want to make into a portrait it's pretty simple there too.
1. Save the Portrait into your portraits subfolder in your NWN folder (see above, if you don't have it make one)
2. Load up the game , preferably not on the character you want to use the portrait for (assuming its a new character and you're not trying to make a new one)
3. Press the ~ button and then type in ConvertPortrait Picturename (replace picturename with the pic's name)
4. That will create a useable portrait under the name of that picture. (Some people like putting a _ after the pic's name when they save it so the files will create as Character_H, Character_L, rather than CharacterH, CharacterL etc)
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Now, if you're asking how to change the pic of an already created character, you put up a General Request post on the forums and specify the name of the portrait you wanna change it to (Even if the Devs don't have the pic saved, they edit your server vault information to reflect the name of the portrait and it'll show up for you.)