For creating a portrait I think most people load the model up in Blender and place a simple shaded / coloured background behind them.
Though you could always use gimp or photoshop to edit it and give it a similar background to the other portraits in the game.
As far as I know portraits are listed in the portraits.2da file
and then referenced by the associated number in .utc files by PortraitID
I use K-GFF Editor and 2DA Editor to access these filetypes, the above is an example of the Yuthura Portrait added in my port of KotOR1 to KotOR2.
This should be everything you need to add a custom portrait besides the making of the portrait itself.
Perhaps you could edit one of these.
Hopefully that helps, you may need to edit it somewhat.