Here is what a ModUnitPortraits.xml file in the mod directory, text folder looks like. I have 75 entries in each but cut it down for posting. These refer to your 128x128 pngs in the citizens folder or GFX folder, not sure which works so I used both.
 
<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<FlavorTextDefs
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="../../Schema/FlavorText.xsd">
<!-- Created with the GC4 editor -->
<!-- UnitPortraits.xml -->
<FlavorTextDef>
    <InternalName>PORTRAIT</InternalName>
    <FlavorTextOption>
      <Requirements>
        <CitizenRace>Race_Dominators</CitizenRace>
      </Requirements>
      <Text>Citizen_Portrait_Dom-01.png</Text>
      <Text>Citizen_Portrait_Dom-02.png</Text>
<Text>Citizen_Portrait_Dom-55.png</Text>
<Text>Citizen_Portrait_Dom-56.png</Text>
      <Text>Citizen_Portrait_Dom-57.png</Text>
      <PreferenceWeight>1</PreferenceWeight>
    </FlavorTextOption>
   <FlavorTextOption>
      <Requirements>
        <GenderMale>0</GenderMale>
        <CitizenRace>Race_Dominators</CitizenRace>
      </Requirements>
  <Text>Citizen_Portrait_Dom-58.png</Text>
  <Text>Citizen_Portrait_Dom-59.png</Text>
  <Text>Citizen_Portrait_Dom-81.png</Text>
  <Text>Citizen_Portrait_Dom-82.png</Text>
      <PreferenceWeight>100</PreferenceWeight>
    </FlavorTextOption>
    </FlavorTextDef>
</FlavorTextDefs>