MLA

Humphry Duke of Gloucester from a Drawing in the Collection of Rich. Bull Esq. Taken from the Honble. Horace Walpole's Original Picture. [Graphic]. Pub. May 1, 1783, by J. Thane, Rupert Street, Hay Market, 0AD. https://collections.library.yale.edu/catalog/16193246.

APA, 6th edition

Humphry Duke of Gloucester from a drawing in the collection of Rich. Bull Esq. taken from the Honble. Horace Walpole's original picture. [graphic]. (0 C.E.). [Portrait prints, Engravings England London 1783, Etchings England London 1783]. Pub. May 1, 1783, by J. Thane, Rupert Street, Hay Market. https://collections.library.yale.edu/catalog/16193246.