Another source of ready-made pixels is stock art. Available on the Web and in collections on CD or DVD, stock artwork consists of photographs, illustrations, clip art, and even audio and motion clips (which cannot be used in Photoshop) prepared by others and licensed for your use. Many programs, including Photoshop, ship with some stock art. (Photoshop CS's default installation includes the Samples folder, inside the Photoshop folder. The stock images are not included with the education version of Photoshop.)
Artwork can also be found on the Internet, in books, and elsewhere. However, you should be aware of copyright laws before using any artwork not created by and for yourself.