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>