Yesterday, during an 'artist's talk' at De Ploegh gallery in Amersfoort, I was asked what the difference was between illustration and 'free' art.
I answered that an illustration was always serving the story, and therefore didn't have to be able to stand on its own.
But a good example that the boundary between illustration and art isn't that firm, is the work of Dutch illustrator and artist Wim Hofman (1941).
His illustrations effortlessly stand on their own, and his art has a certain openness that illustration has also.
Something to aspire to.