In addition to the resources listed below, several library databases include the option to search for images. Be sure to look for images that you are able to use and/or to cite images you use.
Searchable full-text and scanned images of back issues of many of the core research and academic society published journals in almost all subject areas. Its emphasis is in the humanities and social sciences. The majority of JSTOR's content is academic and peer-reviewed. Includes primary source material.
Absorbed Artstor's collection of searchable images from museums and archives across the world. Use the Image tab to search; many of these can be used freely in education and research!
The basic APA citation style for an image retrieved online is as follows:
Artist Last Name, First Initial. Second Initial. (Year). Title of work [Format]. Retrieved from URL (address of web site)
There are going to be variations if there are multiple artists, no artists, no title, etc. Please take a look at the Purdue OWL or the APA Style Manual for more details.