These are databases that primarily contain images rather than articles or other tex-based resources.
Many museums display images of some, or all, of their collections online. Some museums also create online exhibitions with collections of art on certain themes, or from a specific artist or artistic movement.
Images must be cited like all other resources. If you use an image you did not create, you must provide a citation. Images should be cited in all cases, even if the image is very small, or in the public domain. The citation should be accessible in the context of the image's use (within a Powerpoint presentation, on a web page, in a paper, etc.).
Image citations should include the following information at a minimum:
It is also useful to include date, culture, and rights information, if known.
Image citations can be formatted according to the citation style you are using.
For specific examples according to MLA format, see the entry for Paintings, Sculptures & Photographs provided by the OWL at Purdue.
For specific examples according to Chicago Style, see the Citing Images - Chicago Style (Colgate University).