This page outlines how you can use Artstor images in papers, including:
Artstor has a citation generator which will accompany an image. Use it to get citations in APA, MLA, and Chicago format.
All images in Artstor Digital Library are accompanied by metadata--data about the images including things like title, date, and creator--which provides you with the details you need to cite images.
When you view an image in Artstor, the metadata will appear to the right of the image.