A Picture Book is the world in your lap. It can be an art gallery within 32 pages. But it is more than that. It is the love and labor of an author. The words may be few or many but the combination of words and pictures must fit the story.
For example: This page from Caelaach McKinna's book, Little Charley Thornpaws, contains few words. The illustration has been given the job of defining the action. With only one line of text, the illustrator was given an entire two page spread to show the action. Ample space above the running cat was left for that one line.
Some chapter books require much more space for text and very little room for illustrations.
Here the illustrator is challenged to create a meaningful image in a much smaller space and often in black and white.
The many words in a chapter book are important to the author and they need their space along with any illustrations that accompany them. Some illustrations may run along the side of a page or be located only in the top third of the page. Those are normally in black and white but could be color.
In some cases there may be so much text per page that the illustrator needs to use space creatively and sparingly. In the case in a chapter book where only a small portion of a page can be used for an illustration the image need not be more than one single item of importance.
Suspense, mystery, humor, delight, beauty, power, danger, glee and even plain foolishness find their way into an artist's interpretation of the story. But all those emotions are the property of the author first.
An illustrator needs to show respect for the author, the author's vision and the need the author has to create their own world of characters, places, things, adventures.
Painting a two page spread with little regard for the importance of the author's words can lead to a page so filled with images there is little room for text. It may seem like the most basic of rules; the illustrator must leave room for the text.
In addition to the importance of space for the author's words, the illustrator also needs to think of the reader. Leaving room for the text that accompanies an illustration is important. Leaving some "resting space for the reader" is equally important. Not every inch of the page needs paint. White space creates a place for text as well as a rest for the eye of the beholder.
This "resting" space need not be white, but it should be a clam area with not much going on to be defined as true "resting" space.
A well balanced picture book will have illustrations that compliment the author's vision and enough areas of white space or "resting" areas to allow the reader the most enjoyable experience.
The job of the illustrator is to interpret the author's vision, show respect for the words that have been so carefully chosen, and contribute to the journey a reader is about to begin.