An illustrator is an artist who is also a storyteller.
The artsy crowd looked down on "mere illustrators" - that being a person who earned his living by selling his paintings commercially instead of hanging them in galleries and waiting for museums or foundations or rich people to buy them.
Rockwell's easel art is spectacular and shows that he had perfect command of his craft.
You've seen Rosie the Riveter before. Look here:
Another much-despised "illustrator" is N.C. Wyeth, who could paint rings around all his descendants and relations by marriage.