I think there's _something_ to this argument. But it's not the complete picture.
On one hand you have visual art that stands alone, looks impressive, needs no story behind it, other than something only the author knows (a dream, an unwritten story only in the author's head, or an unreleased story, random inspiration).
Then you have art that is designed to represent (scenes or characters from) a written story that already exists. Sometimes it may be what draws the most interest towards a story, other than word of mouth. Probably how half of us got into half the things we've ever read.
So you essentially got both of these, but for different purposes and in different situations. And if it's one or the other you often have to dig deeper. Well, unless it's blatant cover art.