This guide includes great resources for citing many kinds of images in a few different formats. Also, full of links, etc.

A clear description of what is in copyright and what is in the public domain in US.

This is a fantastic and thorough guide from Cornell Library on finding Images. It is absolutely worth exploring for other sources of images.

Duke University Law School has a great set of resources for understanding and using the public domain.

A comic book describing the world of copyright through the eyes of documentary film.

Describes best practices for the fair use of visual resources in educational settings.