'their illustration' is correct and usable in written English.
You can use it to refer to an illustration that belongs to a certain person or group. For example, "The students showed the teacher their illustration of the solar system."
Pierpont Morgan Library, Early Children's Books & Their Illustration (New York, 1975), p. 191.
In some areas, their illustration is probably overly pessimistic because the eclipse may still be viewable through high, thin clouds.
The liberal tilt to the plays is in their overall criticism of the status quo, their illustration of a system that no longer functions.
We shall discuss and analyze a variety of data structures and algorithms chosen for their importance and their illustration of fundamental concepts.
Partly because of such collaborations, and partly because children's books provided a hiding place for a while, the early Soviet period was a miraculously rich time for children's books and their illustration.
Also notable is a work by the best-known composer on hand, H. I. F. Biber, who pushed the Stylus Phantasticus to its symbolic limits in works such as "The Presentation in the Temple," the fourth of his "Mystery Sonatas," which, with their illustration of New Testament episodes, add a narrative dimension to the spectacle of expressive performance.
"For too long, fashion illustrators and their illustrations were seen as a secondary art form — rooted in the idea that decorative art, or art with function, is somehow lesser," Ms. Gray said.
The first edition of The Arabian Nights had no pictures, and even when, in the late 18th century, fully illustrated editions began to be published, their illustrations gave little sense of the exotic medieval Arab environment in which the stories were set.
They print their illustrations rather well.
Some of the painters themselves offer simple explanations for the appeal of their illustrations.
While Egyptians watched the news Wednesday night, cartoonists were preparing their illustrations for Thursday.
