Where does the word illustration originate from. Often words come from a very different origin then to which it is applicable today.

Etymology of Illustration:

c.1375, “a spiritual illumination,” from O.Fr. illustration, from L. illustrationem (nom. illustratio) “vivid representation” (in writing), lit. “an enlightening,” from illustrare “light up, embellish, distinguish,” from in- “in” + lustrare “make bright, illuminate.” Mental sense of “act of making clear in the mind” is from 1581. Meaning “an illustrative picture” is from 1816. Illustrate “educate by means of examples,” first recorded 1612. Sense of “provide pictures to explain or decorate” is 1638.

This was taken from etymonline.com.

So we looking at “A spiritual illumination”, an enlightening, distinguish and clear in mind etc… all sounding good to me! No hidden darker agenda behind the word.