This page outlines how you can use Artstor images in papers, including:
Citing images can be tricky. The Library of Congress provides a great overview of how to cite images/primary source materials like paintings, photographs, and documents. Read more here: http://www.loc.gov/teachers/usingprimarysources/citing.html
Your university's librarians are a tremendous resource for learning how to cite images and other sources--reach out to them with further questions!