Images can be acquired by using a camera, scanner, clip-art and stock photography, or from the web. Most film developers offer digital processing, which converts your 35mm film or existing prints into digital images stored on a CD. However, if you plan to take a lot of photographs for your site you might want to consider buying a digital camera, which skips the developing time and expenses.
For existing prints, a scanner is a good way to digitize them. See webmonkey's scanner tutorial for more information on scanning images properly for web use.
Scanner tutorial: http://hotwired.lycos.com/webmonkey/geektalk/97/41/index3a.html
Another great place to find graphics for the Web is the Web itself. Saving pictures from a website is as easy as right clicking on the image (ALT + click for Macs) and selecting "Save Picture As". However, most online content is copyrighted material. To use an image from the Web, you must first get permission from the original owner. When in doubt, assume an image is copyrighted unless explicitly stated otherwise. Obvious violations of copyrighted material are subject to removal from the DePauw website. The DePauw Photo Database was created to address this issue, and provides a variety of images that you can use freely in your site.
Publications Photo Gallery: http://www.depauw.edu/univ/publications/albums.asp
DePauw Photo Database: http://www.depauw.edu/photos
Several online resources offer free clip art and stock photography, although the quality of what's available varies dramatically from source to source.
http://www.1clipart.com/
http://www.free-clip-art.com/
http://www.gifart.com/