This section specifically means portraits in portraits.2da that are not tied to a particular creature appearance. Those are usually found with the creature themselves.
The base game ones will get a 4x upscale and any new ones will be listed here.
To follow. Need to pick out which ones are appearance.2da related.
To follow.
Naming convention would be: po_op_XXX_t.dds (where T would be the appropriate size ID). XXX can be 9 characters long. This is because of portraits.2da limitations (you are forced to use po_ as a prefix!)