An illustration is a work of art. It's an idea that may be particular to understand. The work of art can be expressed in the creation of an illustration stemming from a commission, but also in the creation of an illustration at the sole initiative of the creator. I'm thinking of children's books, which may be the initiative of a writer-illustrator who decides to write a book and illustrate it. As such, it is a work of art subject to copyright, as are the works of all visual artists.
On January 31st, 2019. See this statement in context.