Copyright is a form of protection provided by the laws of the United States (title 17, U.S.Code) to the authors of “original works of authorship,” including literary, dramatic, musical, artistic, and certain other intellectual works. This protection is available to both published and unpublished works.
from US Copyright Office "Copyright Basics"
Even if you can use images for free, most images require that you credit the copyright holder or the website where you got the image from. Here are some sites to get you started:
Check out this 10 minute video that covers copyright basics and Fair Use - made entirely with Disney clips!