MLA

Christina of Lorrain, Great Duchess of Tuscany [Art Original] : Wife of Ferdinand the Great, 3d. Gr. Duke. 0AD. https://collections.library.yale.edu/catalog/11134635.

APA, 6th edition

Christina of Lorrain, Great Duchess of Tuscany [art original] : wife of Ferdinand the Great, 3d. Gr. Duke. (0 C.E.). [Portraits, Watercolors 18th century]. https://collections.library.yale.edu/catalog/11134635.