You can add any tags you want for users, but the code currently only check for a limited set. "Tags" according to the game refers to any string of text found in the file path or name.
The unmodded game will only search for any word of the race to be present in the tags, and will randomly choose any picture that qualifies. If you add multiple races to the tags, it should work for any of them.
The Improved Random Portraits mod uses filters and weighted tags when selecting a random picture. According to the info file it uses(check file for supported tags): Race, Gender, Age, Hair Color, Skin Color, Tit Size, Ass Size. I haven't looked into specific mechanics.