MLA

Shelley, Samuel, approximately 1750-1808, artist. [Unknown gentleman in maroon coat and lace cravat] [art original].. [18th century?]. https://collections.library.yale.edu/catalog/15815227.

APA, 6th edition

Shelley, Samuel, approximately 1750-1808, artist. ([18th century?])[[Unknown gentleman in maroon coat and lace cravat] [art original].]. https://collections.library.yale.edu/catalog/15815227.