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Portraits are made up of several different files all with the same prefix - po_ - but with different end letters. Generally TGA files are used for these to retain quality but DDS tends to work better because of the way save games generate thumbnails if you are using scaled up resolutions (apparently TGA files being too big corrupts the save games generating the little thumbnails for the load menu, while DDS does not).
You can convert files in the engine itself using the command ConvertPortrait filename (if present in the "portraits" folder) but to be honest, you will find it easier to edit files in something like Paint.NET or GIMP to get the required sizes since you can easily crop to retain detail in the smaller sizes.
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