At the beginning of the movie after she gets her book and she sits down at the fountain and the sheep takes a bite out of a page in the book. You can clearly see that the sheep takes a bite out of a page with a picture on it. Later, Gaston says, "How can you read this? It doesn't even have any pictures!" I thought it was a cute slip-up!
After Gaston says that, Belle says that you have to use your imagination. And they were just showing what Belle's imagination was imagining it to be like in a picture.
This isn't a slip up, Gaston flicks through the book, so quickly that he probably only saw writing. We all know that Gaston isn't into books, so when Gaston says "How can you read this, there aren't any pictures", he probably means, there's only a few pictures, and heaps of writing.
I never noticed that before. That's interesting. What I did notice was how much the girl in the picture looked like Belle. And it seemed that she was reading Beauty and the Beast.