After reading this question it made me wonder: are there any instances where the illustrations in the Illustrated Harry Potter series contradict known book canon?
Update: I misunderstood the question. (I did not realize it was supposed to be about the illustrated editions, specifically.) However, I am leaving this answer for posterity.
The covers of the first American editions of the seven Harry Potter novels all depict events that occur in the books (albeit with some graphical distortion and extra stuff going on in the background)—except for the cover of Harry Potter and the Goblet of Fire.
It can actually be quite jarring to look at this cover, side by side with Mary GrandPré's other covers. Rather than depicting any kind of action scene from the book, this one is just a collage of imagery. It shows the four participants in the Triwizard Tournament, all together inside the hedge maze, along with a number of other iconic elements from the plot—a panorama that never occurs in the story.
I presume that the reason for change in the art was not a creative decision by Ms. GrandPré, but that it was likely mandated by the publisher. Goblet of Fire was the first Harry Potter book to come out after they had become a huge international sensation, and the executives appear to have mandated that the protagonist should be present and more centrally located in the cover art from that point forward. For the fifth, sixth, and seventh volumes, though, the cover illustrations are back to being action scenes from the stories, although the keep Harry himself large and near the center of the picture.