In the Oxford English Dictionary (OED), the noun ‘illustration’ makes for some interesting reading. Broadly, current usage of the term is divided into an action and a material object – the image. For the former, two senses are given:
‘The action of making or fact of being made illustrious, brilliant, or distinguished; distinction. Also, An example, means or cause of distinction.’
This was first used in 1616, is still employed and example quotations are given from the mid nineteenth century. Then, there is this:
‘The action or fact of making clear or evident to the mind’
This sense was first used, according to the OED, in 1581. John Ruskin wrote in the second volume of his Stones of Venice in 1853: ‘I have confined the illustration of it to architecture’ (II. vi. 170), meaning that something was to be explained by architecture itself, rather than through his words (or illustrations). Illustration seems to have acquired a material, or pictorial, sense only in the nineteenth century, according to the OED. From 1813 the following sense is recorded:
‘The pictorial elucidation of any subject; the elucidation or embellishment of a literary or scientific article, book, etc., by pictorial representations.’
And then, emerging at around the same time:
‘An illustrative picture; a drawing, plate, engraving, cut, or the like, illustrating or embellishing a literary article, a book, etc.’
John Ruskin, again, for instance, subtitles his Stones of Venice with the addition ‘with Illustrations drawn by the Author’, clearly referring to his own drawings, rather than the implicit meaning of architecture. Of course, when ‘illustration’ is used in its pictorial sense, some of the action of ‘making clear or evident to the mind’ survives. Illustrative images explain something else – most commonly texts. As Tom Gretton has remarked, ‘When pictures are made, or received, as ‘illustrations’, they are understood to be motivated by and at the service of texts’. However, Gretton also argues that the images printed in the Illustrated London News and other illustrated journals very quickly lost this serving character, gaining independence from, and eventually dominance over, the accompanying text. This was expressed, he claims, through the way in which images were laid out on the page, ignoring the three-column formula of the text, forcing upon the verbal part of the Illustrated London News’ pages a plasticity that formerly only the images themselves had had to show.
 Tom Gretton, ‘The Pragmatics of Page Design in Nineteenth-Century General-Interest Weekly Illustrated News Magazines in London and Paris’, Art History, 33 (2010), 680–709 (p. 683).