If you say you imagined her like the illustrations from the book, then you didn't imagine her at all. You let the illustrations imagine everything for you.
Just because JK Rowling approved the illustrations (and I'm not sure she did, because often, VERY often, authors have no say in who is chosen for the illustrations or in the artwork itself---especially with a then-unknown author), it doesn't mean this is her only vision of how the character might look. She has approved a black Hermione herself.
If we were locked into the illustrations in a book, then Margaret Hamilton never would have looked the way she did in the iconic role of the Wicked Witch of the West. The book illustrations look nothing like that.
Vivien Leigh never would have played Scarlett O'Hara. The opening line of the book is "Scarlett O'Hara was not pretty ..."
As an author myself, I do have a vision in my mind of what a character looks like, but I also realize that when the book is published, I am releasing that character into the minds of readers. That character is no longer "mine," living solely in my own imagination. I am sharing that character with each individual, and the visions of everyone are never the same exactly (or even remotely).
Open up your mind, and leave your prejudices at the door.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22