Ok:
1) Please bear in mind that if you are releasing this for public use, there may be copyright or DLC-locking issues relating to the use of edited versions of DLC graphics files.
2) The .dds files must include mipmaps and use the correct compression format.
3) The .dds files are just part of the portrait assembly.
4) The .gfx files in interface/portraits (which are just text files, editable in Notepad++) specify which .dds files are used in each sprite sheet, and which sprite sheets are used in each layer for each ethnicity.
5) The file or files in common/graphicalculturetypes define which ethnicities/portrait sets are valid, along with a skin tone code to enable a sensible choice to be made for mixed-race characters.
6) The culture files in common/cultures specify which portrait sets belong to which cultures.
7) The file in interface/portrait_properties controls when a character is given specific optional features such as different clothes.
8) There's a whole other bit if you want to override Merchant Republic garb.
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