In the Strive game folder, the file "files/scripts/characters/races.gd" can be opened with a text editor. It will have the names of all the races except Halfkin, which are simply the races with "Beastkin" replaced with "Halfkin". The game searches the file name and relative file path from the "portraits" folder for either on of the words found in a race name. If either is found it adds it to a pool of candidate images and will later randomly assign one to the slave. Also it ignores the case of the letters, 'B' == 'b'. So a "Beastkin Bunny" slave will be assigned a picture contain either "beastkin" or "bunny".
My earlier post is the entire tag system for vanilla Strive, there are no other tags. There is no system for automatically replacing NPC art, though you could probably mod the gallery.gd file to use other images. Any slave can have its images replaced manually, so once you acquire an NPC you can replace their images, though that won't effect the quest images.
If you are looking for further tag support, then you will need to explore the improved random portraits mod (https://itch.io/t/281535/improved-random-portraits-v10). It still has some flaws as articulated in the last few comments, but works fairly well. It has a guide included in its files.
This is probably the info he was linking to in the discord(edit: there is a channel where people discuss portrait packs):
the slave tab portraits are 139x139
the slave tab full bodies are 370x608
the inventory panel's bodies are 376x430
combat portraits are 105x105, though enemies are 90x90(edited)
for sex interaction portraits are 35x35, and bodies are 600x600
thus the slave tab dimensions are basically the max you can use(unless you want something grand for the sex portrait), anything different is fine as it should keep aspect, though it will shrink/expand to fit
