So, I can't find an exact answer to this, but what is the criteria for adding in pictures for portraits and body pics?
The basic criteria for adding pictures is having the picture file and knowing where to find it. Perhaps you meant something else, but you didn't elaborate so it's hard to know.
If you meant file types: png, jpg, webp.
If you meant sizes, any size will work, but images that match the width to height of the containers will look better. Portraits should be 139x139(square), bodies should be 370x608(~60% taller than wide).
If you meant where does the game look for pictures by default, then (for Windows) you will find them in the game's app data folder in the Roaming folder.
If you meant assigning matching portrait and body pictures, then they need to have the same relative path(from portraits or bodies) and file name.
If you meant tagging the files so chooses images that match the race of a character, then the relative path or file name need to contain at least one word of the race name as found in "scripts/characters/races.gd". (beastkin races double as halfkin races) Further tags are only available using the improved portraits mod.