For academic purposes sources for images must be cited even if free or in the public domain.
The basic information you will need:
- Artist's name
- Title of work
- Date it was created
- Repository, museum or owner
- Country of origin
- Dimensions
- Material or medium of work not image (oil on canvas, photograph, marble, etc.)
Citation Styles are: MLA, APA, Turabain, Chicago Manual of Style