portraits.2da is a text file with the following format:
http://nwn.wikia.com...i/Portraits.2da
Use NWNExplorer to export the version in your top hak (if you don't have one, the most recent version from the OC). 2da files appear under Game Data.
Add new lines with a text editor, use nwnhak to put the new version in your top hak.
Your portraits need to be in a specific format:
http://www.neverwint...o/portraits.htm
The portraits also go in your custom hak.