US Copyright Office: How to Investigate the Copyright Status of a Work (PDF)
The Artstor Digital Library is an extensive and representative visual resource of images for teaching and research in the arts, humanities and social sciences used in teaching and learning contexts. Thus, the Digital Library is characterized by high-quality images and authoritative cataloging information.
Provides free licenses and other legal tools to mark creative work with the freedom the creator wants it to carry, so others can share, remix, use commercially, or any combination.
When you need to purchase rights to publish a copyrighted image, look here: