Paintings, Sculptures, Photographs (viewed on Artstor)
In Artstor, look for "Cite" to get the citation:

Paintings, Sculptures, Photographs (viewed on museum website)
Last Name, First Name. Title. Publication year. Name of Museum or Institution where artwork is displayed, Website URL, Date of Access.
URLs should not include http:// or https:// in your citation. Your professor may ask you to omit URLs for neatness.
Kandinsky, Vasily. Picture with an Archer. 1909. Museum of Modern Art, www.moma.org/collection/works/80104, Accessed 8 February 2019.
Paintings, Sculptures, Photographs (viewed on a website other than the museum's)
Last Name, First Name. Title. Publication year. Name of Museum or Institution where the artwork is displayed, Location of Museum/Institution, Name of Website where
artwork is displayed, website URL, Date of Access.
URLs should not include http:// or https:// in your citation. Your professor may ask you to omit URLs for neatness.
Klee, Paul. Twittering Machine. 1922. Museum of Modern Art, New York. The Artchive, www.artchive.com/artchive/K/klee/twittering_machine.jpg.html. 11 February 2019.