Copyright registration for works of the visual arts (United States Copyright Office)
Copyright registration of photographs (United States Copyright Office)
"I have a photograph that I took, and there are people in it. Do I need releases from them before I can use my own photograph?" http://copyright.princeton.edu/faqs/
If you are incorporating images created by other artists into your own artwork: see "Making Art" in http://www.collegeart.org/fair-use/
