The following tools will help you answer these questions:
Includes still images, videos and sounds with Creative Commons licensing.
Provides an extensive listing of resources under the following headings:
Creative Commons Meta Search: You should always verify that the work you are re-using has a Creative Commmons license attached to it.
How to apply licensing filters and provide attribution:
Stock images are usually found in commercial collections in which case the image can be purchased for a fee. However, there are many free collections as well. Additional collections can be identified through normal search engines, and by subject ie) Engineering Stock images.