Portraits are located in the following directories:

<install-directory>\PillarsOfEternityII_Data\gui\portraits\player\<male|female>

Portrait names, sizes, and styles are:

<male|female>_<name>_<a-z>_convo.png - 90 x 141 (ink)
<male|female>_<name>_<a-z>_lg.png - 210 x 330 (watercolor)
<male|female>_<name>_<a-z>_si.png - 76 x 96 (ink)
<male|female>_<name>_<a-z>_sm.png - 76 x 96 (watercolor)