Canon Illustrations

Sometimes one set of illustrations for a book or series, especially a classic, becomes so ubiquitous that those images become our cultural idea of what the scene or character "looks like," regardless of how it's described in the text.

There are three ways this can work. First, the illustrations may accurately follow the text. Second, the illustrations may include prominent details that don't contradict the text, but also aren't mentioned. Third, the illustrations may outright contradict the textual descriptions.

Examples, by their relationship to canon:

Not contradicting the text

Literature

John R. Neill's illustrations for the early Land of Oz books.
Sir John Tenniel's illustrations for Lewis Carroll's works, particularly the Alice in Wonderland books.
Antoine de Saint-Exupery's The Little Prince, (self-admittedly poorly) drawn by the author himself. Re-illustrated versions are heresy.
Garth Williams's illustrations of Little House on the Prairie, Charlotte's Web and other children's books.
The Polar Express is famous for its gorgeous, full-page illustrations, most of which were reproduced in The Film of the Book (it was probably the point of making a movie to begin with).
Paul Kidby's The Pratchett Portfolio, and subsequent Discworld covers.

Western Animation

The Disney animated version of Winnie the Pooh has taken over from the E. H. Shepard illustrations as the "official" look for the characters, but recently, the Shepard art style has gained a resurgence as the "Classic Pooh" brand extension.

Adding important details not mentioned in the text

Literature

Sherlock Holmes and his deerstalker hat, added by Strand artist Sidney Paget (but only when Holmes was in the countryside).
In the Magic School Bus books, the text and illustrations complement each other, with the illustrations and dialogue frequently including details the text leaves out. For example, the text of most of the books doesn't mention any of the children's names, but they are all worked into the dialogue.
Going back to Tenniel and Alice in Wonderland, the price tag on the Mad Hatter's hat ("In This Style 10/6", the fraction being ten shillings and sixpence in the old monetary system) was Tenniel's own invention and nowhere to be found on the text. It has been retained in some form or another in almost all adaptations.
Snape is never described as having a beard in Harry Potter, but the goatee Mary Grandpre gave him became Fanon. However, it's been largely superseded by Alan Rickman's portrayal of him in the movies.
- Before the films, a lot of Harry Potter merchandise seemed to copy the look of Mary Grandpre's illustrations, including kind-of-sort-of keeping her "soft geometry" art style.
Paul Kidby's illustrations to The Last Hero, adding such details as Ponder's "Actually I Am A Rocket Wizard" Fun T-Shirt, now as ubiquitous to the character as the robe that looks like an old-fashioned anorak (which is in the text). Defictionalization has occured.

Tabletop Games

For many Dark Sun players, Brom provided the definitive look for Athas with his novel and sourcebook covers. When the Fourth Edition campaign guide came out, many called for Wizards of the Coast to hire him for the cover. They didn't.

Contradict the text

For instances of lying book covers, see Covers Always Lie.

Literature

Heidi is described as having short, black, curly hair.
W.W. Denslow's illustrations for The Wonderful Wizard of Oz show a dark-haired Dorothy, while John R. Neill's later illustrations showed Dorothy as a blonde. In a case of First Installment Wins, the Metro-Goldwyn-Mayer film followed the Denslow illustrations, and Adaptation Displacement did the rest.
The original illustrations from Conan the Barbarian's magazine serialization depict him as the now-stereotypical Loin Cloth-clad brute, but in the actual text what Conan felt to be one of the advantages of his stature was the ability to wear very heavy armor (usually chainmail over leather) while remaining agile and unencumbered.
The Dark Tower books... in some of them, Jake is described as having blonde hair, but in one illustration has black hair.
The Pauline Baynes illustrations of The Chronicles of Narnia. She contradicted Lucy's Hair of Gold, but people still believed in her chosen colors for the rest of the (never-described) kids enough to complain about the Adaptation Dye Job when the recent movies gave Peter blond hair and Edmund brown/black hair, the reverse of how she drew them.