As a portrait artist I can tell you that The TEETH need to be shaded on either side near the corners of the mouth , otherwise the characters have a strangely OFF look detracting from the realistic appearence. This is a basic principle of portrait painting and not shading is a common mistake of students.
I suggest that the teeth mot AL be bright white, but shaded near the corners of the mouth. Then the avatars will look more realistic. ALso the "Whites" of the eyes should NOT be actually White, but a VERY LIGHT shade (alomost white) of the skin color. Again this will remove the "spooky white" look of the eyes and make the avatars look more real still.