How do you format a picture for a portrait in NWN? I've got one for Thira that actually matches her description

Basically, you want to create 5 different image files in .tga format. You can use something free like paint.net (found here:
www.getpaint.net/index.html ), though others will work (that's just the one I use).
The files need to be formatted to a certain size, and would be something like:
thiraportrait_H.tga 256x512
thiraportrait_L.tga 128x256
thiraportrait_M.tga 64x128
thiraportrait_S.tga 32x64
thiraportrait_T.tga 16x32
Note that you can make these resized or cropped as you see fit, the game doesn't care if they match or not, just that they all use the same root filename with the size modifier on the end.
and these go in my override folder right?
Be careful with the portrait size. The lower portion of the portrait isn't visible.
nwn.wikia.com/wiki/Portrait has the sizes of the visible portion and the total size of the portrait.
You can put them in portrait or override. The ones in the former are available for use when creating new character, the latter aren't. However, when you reach a certain number of portraits, NWN will start crashing whenever you get to that point of character creation, so it eventually gets necessary to put some of them in override.
Also, you need to post a link to the file so we can all download it, so we can see the pictures too.
I put it up in the LETO forum, you can totally download them from there

Is there a permanent portrait archive?
Yep, in the Character Biography forum, there's a thread.
There's also a very simple procedure to create a set of NWN portraits from a single jpg file. This will save you all of the trouble of getting the formatting of portraits right, and avoid the crash producing errors that can sometimes happen:
1) Place the portrait you want to use in jpg format in your NWN "portraits" folder. Proportions should be a multiple of 64 pixels wide by 100 pixels high for the whole image to be used, but close to that will work and just be cropped a little. Resolution of at least 256 wide by 400 high (the resolution used in the largest of five portraits) will look good in all sizes. Other resolutions will be resized as needed. Be sure to note the jpg image's filename - you'll need to type it exactly in the last step.
2) Open the NWN player client in either multi or single player with any character.
3) Press the "~" key, which will give you the ">" symbol for text entry at the upper left of your player window.
4) Type "Conv" (without the quotes) and press the TAB key. You should see "ConvertPortrait _"
5) Type your original portrait filename exactly, minus the .jpg extension, and press ENTER. You will see a message that the portraits have been created, and will find the set of five in your NWN portraits folder. ("mycharacter.jpg" will produce "mycharacterH.tga" through "mycharacterT.tga")
It does work, but it doesn't zoom in on specific faces, so... sometimes it doesn't do the proportions well.
File names also need to be 16 characters or less.
Just one of those odd quirks of NWN.
What I would love to know is how to best create this unused portion of a portrait.
White post the picture you want I'll try to crop it for you.
Oh thank you so much I was only generally curious

The way I do it is to crop the first picture. Then I create a blank new picture at the bigger size, and copy/paste (or insert) the original picture onto it.
It does work, but it doesn't zoom in on specific faces, so... sometimes it doesn't do the proportions well.
One way I have gotten around this when the larger image(s) is a larger view, such as partial or whole body, but the smaller images are just the character's face, is to create one set for the large image, and a second set from a jpg that is pre-cropped to just the face as desired, then rename.
My go to portrait creation is with fixed selection sizes. Makes it really easy to get just the right size/selection of any photo by decreasing the picture size (As long as it is larger than the NWN portrait size needed. Which is rare to not be the case.) And then you can add the blank space with canvas size afterward. Rinse and repeat if you need want to make the different portrait sizes zoom differently.