‘Concept art’ typically refers to visual art when it’s being used to represent individual elements that are intended to become a part of a broader, more elaborate visual composition (Wikipedia definition: “a form of illustration used to convey an idea for use in films, video games, animation, comic books or other media before it is put into final production”). The key element appears to be the artwork’s practical application, for the creation of something more elaborate, or maybe the essential feature is an inherent reference to some bigger artistic output, or perhaps just that the concept art contain a reference to a story.
What is evident is that concept art is not an independent art form. If we imagine a spectrum of various art forms where concept art (not to be confused with conceptual art) would be included, its polar opposite would logically be the abstract art, as the presence of abstract artwork (in any given context) doesn’t require justification through clear reference to anything that’s known to anyone other than the artist behind the work in question. On the other hand, the key criteria for something to be considered concept art is that it exist primarily as reference to something else and, moreover, (usually) in reference to something that has yet to be created in a finalized version at the time that the concept art is created.
With bloggers becoming ever more dependent on images to attract attention to their articles, at the same time as the well of available stock photos may be running dry, those online content creators will become increasingly dependent on coming up with clever new ways of drawing attention to their work. With the very real possibility of being threatened with legal action for featuring a copyrighted image without permission, the advantage of being able to produce one’s own original artwork is undeniable. Illustrations that are created for the express purpose of being featured alongside a text should, in their own right, be considered concept art, much the same as characters that are designed to be featured in a video game, cartoon or a comic book.
I would suggest broadening the definition of concept art even further, so that the articles themselves should be considered to be concept art, as long as they are presented as reference to a more elaborate system of ideas than can reasonably be contained within the length of the text, thus qualifying as literary concept art.