Illustration
An illustration is a drawing, photograph or other type of image placed to support the text of a book, article, web page or advertisement. On the one hand, an illustration makes it clear before reading what the text is about. On the other hand, the illustration serves as an explanation after reading (part of) the text.
The illustration by itself may contain information or additional explanation, as in an infographic. Illustrations can also have a more decorative or contemplative function, as in a cartoon or stock photo.
Someone who provides illustrations for a book is called an illustrator.