To cite the cover of a magazine, you can generally create a works-cited-list entry for the issue of the magazine and then key your in-text reference to the first element of the entry:
The most recent issue of The Nation features on its cover an image of a donkey with the top of the Capitol building on its back.
The Nation. 17-24 Dec. 2018, www.thenation.com/issue/december-17-24-2018-issue/.
If you discuss a cover image in detail and wish 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.
Published 19 April 2019