Incunabula Maurits Sabbe Library
Illustrations can be printed or painted as well. Woodcut diagrams could be used to clarify scientific works, when necessary. Images could depict scenes from the story, or be used purely as decoration, just like in modern books. These illustrations were usually carefully planned beforehand, but it's clear that even five centuries ago, people were sometimes of the opinion that a book didn't have enough pictures. And in those cases one has to take matters (or a pen) into their own hands*.
* Please refrain from actually writing or drawing in library books.