An Illustration is a visualization such as a drawing, painting, photograph or other work of art that stresses subject more than form. The aim of an illustration is to elucidate or decorate a story, poem or piece of textual information (such as a newspaper article), traditionally by providing a visual representation of something described in the text.
(wikipidia.org)

Imagination is accepted as the innate ability and process to invent partial or complete personal realms within the mind from elements derived from sense perceptions of the shared world. The term is technically used in psychology for the process of reviving in the mind percepts of objects formerly given in sense perception. Since this use of the term conflicts with that of ordinary language, some psychologists have preferred to describe this process as "imaging" or "imagery" or to speak of it as "reproductive" as opposed to "productive" or "constructive" imagination. Imagined images are seen with the "mind's eye".

One hypothesis for the evolution of human imagination is that it allowed conscious beings to solve problems (and hence increase an individual's fitness) by use of mental simulation.
( wikipidia.org )