The derivative examples he provides below are copies of truly iconic images, while Miles Davis cover is a pretty mediocre photo by a commercial photographer.
To your second point; the original photograph on the Miles Davis cover has absolutely become an iconic piece of American culture - I take it you're not a jazz enthusiast?
And then go search for "morrison".
And 20th for "Miles Davis": http://images.google.com/search?hl=en&biw=1400&bih=9...
"Kind of Blue" the album is incredibly famous and respected (http://en.wikipedia.org/wiki/Kind_of_Blue#Reception_and_infl...), and its album art--though not the best I've seen--is therefore a Big Deal by association, if nothing else.