The phrase "illustration picture" refers to an image that helps explain or demonstrate something. It is usually a visual representation used to give an idea or example of what is being talked about. Full definition
I'm literally going back to school and the book gives clear illustration pictures directions and why the procedure is formed so you can inform the pt. (amazon.com)
This illustration picture taken on April 19, 2018 shows the logo of the Itunes app of Apple displayed on a tablet screen in Paris. (fortune.com)
FILE PHOTO: A Bitcoin (virtual currency) coin is seen in an illustration picture taken at La Maison du Bitcoin in Paris, France, June 23, 2017. (reuters.com)