Art images are readily accessible, but it is important to determine whether an image is in the public domain or whether the image is copyrighted. If the image is copyrighted, permission from the artist or owner is required.
Art history courses generally use Chicago Manual of Style for image citation. However, check with your instructor to see what style they prefer.
See these image citation resources for a variety of citation styles from Purdue University's Online Writing Lab (OWL):