Although not very common in LOTR cover art, Tolkien made drawings to represent his ideas about Middle Earth.
Tolkien also created a cover design that features a graphic ring surrounded by Elvish writing. Other elements have been added to the basic design. Tolkien's original cover illustrations can be seen here.
For the Hobbit, Tolkien drew a landscape that wraps around the back, spine, and front covers and features Smaug, the eagles, and several locations in the story.
Christopher Tolkien redrew his father's draft map in 1953 and the elder Tolkien used this map as a reference while writing.