So what makes something “fine art”?
Mastery and critical acclaim, just as for music and literature.
“So what makes something “fine art”?”
It’s whatever you can’t afford, can’t understand, or can’t fit above your mantle.
I'm a professional illustrator, so you can take the following FWIW.
With fine art, you have to consider theme, composition, harmony, expression, etc. Illustration can be as simple as a form of photography. An example of pure illustration would be a painting of an apple for a Smuckers label.
Rockwell did both. An example of his fine art would be the "Thanksgiving Meal." There are numerous examples of his more illustrator-y, commercial art. And there is a continuum from one to the other.