Graphic Novels are typically cited using the standard MLA book format:
Author(s). Title of Book. Other Contributor(s), Publisher, Year.
Here is an example of a graphic novel created entirely by one person:
Barry, Lynda. What It Is. Drawn & Quarterly, 2008.
Here is an example of a graphic novel with multiple contributors. List their names, preceded by a description of their role:
Tamaki, Mariko. This One Summer. Illustrated by Jillian Tamaki, First Second,
2014.