I think that all of the illustrators used watercolor and engraving as media. A departure from that Art neuveau/Arts n Crafts style was Arthur Rachlans more solid figures . Ealriest 20th century printing still had relied on engraving and color engraving. The earliest WITW version was maybe in the first decade of the 20th, Rachlan was putting the touches onto his illustrations in 1938. Ill bet there are over 50 different illustrators in the various versions Of WITW, some of them as recent as 2007.
I think that even Murray Tinkleman did a version of the illustrations.