Note, all advice I provide is for the first Strive, not Conquest.
Basic image info can be found here: https://itch.io/t/838806/core-tag-size-rules
The "WolfGirl" advice is likely outdated as there is very little functionality to that text, though vanilla random portraits will use the "Wolf" for marching either the beastkin or halfkin. It's better to match existing file and folder naming conventions and add unique identifiers(could be as simple as a number) when file names collide.
Any images in the "portraits" or "bodies" folders will be available in game. When the paths and file names of two images matches except for being in the opposite folder, they will be considered paired and often be assigned together. Images are often placed in folders to tag images for use with the Improved Random Portraits mod.
As far as the images not being visible in game, I have very little idea. I assume this means that you assigned then to slaves and can see nothing. If that is the case then I would assume that the .png files are saved using the wrong color profile so Godot is finding a blank image. Normally this leaves warnings in the debug logs located in AppData, or you could try the Debug mod.