MLA

Willyams, Cooper, 1762-1816, artist. [Manfred watching servants lift the giant helmet off his dead son] [art original]. [April 1787]. https://collections.library.yale.edu/catalog/16704386.

APA, 6th edition

Willyams, Cooper, 1762-1816, artist. ([April 1787])[[Manfred watching servants lift the giant helmet off his dead son] [art original]]. https://collections.library.yale.edu/catalog/16704386.