To cite the cover of a book, create a works-cited-list entry for the book and then key your in-text reference to the first element of the entry: 

The cover of Where the Wild Things Are by Maurice Sendak depicts a large horned creature sitting in a forest next to a body of water and a boat. 

Work Cited

Sendak, Maurice. Where the Wild Things Are. HarperCollins Publishers, 25th anniversary ed., 1963.

If you discuss a cover image in detail and want to credit the artist, you could provide the artist’s full name at first mention in your prose or the artist’s last name in parentheses and list the entry under the artist’s name. Note that in this example, the illustration on the cover of the book is by the author of the book.

The cover of Where the Wild Things Are features an illustration, created by Maurice Sendak, of a large horned creature sitting in a forest next to a body of water and a boat.

Work Cited

Sendak, Maurice. Cover image. Where the Wild Things Are, by Sendak, HarperCollins Publishers, 25th anniversary ed., 1963.

or

The cover of Where the Wild Things Are features an illustration of a large horned creature sitting in a forest next to a body of water and a boat (Sendak).

Work Cited

Sendak, Maurice. Cover image. Where the Wild Things Are, by Sendak, HarperCollins Publishers, 25th anniversary ed., 1963.

For citing magazine cover art, see this post.