The purpose of thumbnail sketches is to advance the concept of artist, art director and editor to a final product. It seems like a lot of busy work, but three elements are essential: brainstorming, mind-mapping and closing the gap. The following images illustrate the process of thumbnail sketches as it relates to a book cover illustration. Three thumbnail cover comps presented to the publisher a couple months back. Full-size book cover sketch to gauge color temperature and composition of elements.