Remember, you need to cite any and all images you use, just like you would a book or article. For guidelines on how to appropriately attribute and cite an image, use the resources below.
From the University of Cincinnati, this website outlines image citation for each citation style (MLA, APA, etc.).
MLA Citation Style: OWL @ Purdue
Scroll down to "Painting" to see examples of cited artwork in MLA style.
A research guide dedicated to visual literacy from UC Irvine, this page and its links provides some great information about copyright, attribution, and best practices for citing images.
From Simon Fraser University, this guide shares information on how to find and use images, including citation and attribution.