First of all, always start with the assumption that any image you find on the web is copyrighted. Any digital image will have been created in the last few decades at most. There will be some that are simply copies of public domain materials, and so might not have the originality necessary for a separate copyright. However, it is best to start with the assumption of copyright protection.
Just because something is copyrighted (when did that become a verb, anyway?) doesn't mean that you can't use it. There are lots of sites that specialize in providing images for personal and educational use, for free. Also, fair use covers a lot of educational usages. However, starting with the more permissive sites may be simpler, and lays the groundwork for future use when you may not be covered by fair use (for instance, a job-related presentation or commercial website.)
While Google Images and Yahoo! Images are great search engines for images, they are not the best place to start for images that you want to use. There is no distinction between material that you can use and material you can't.
One of the first places I'd start would be the Creative Commons search page. Creative Commons is a "copyleft" organization, dedicated to providing a method for creators to retain ownership of their copyrights while still providing easy terms of usage. Sort of a clearinghouse for those who believe that "imitation is the sincerest form of flattery" is more true than "time is money". On the search page, you can limit your search to images if you use the Nutch search. Remember that, like most image searches, you are actually searching the words on the page that contains an image and so will likely have a lot of irrelevant results. For instance, a search for "librarian" limited to images, turns up every picture on a blog with links to the Filipino Librarian blog, despite the fact that none of the pictures (or posts) on the first blog have anything to do with librarians.
That's the first lesson for image searching--there is a fundamental mismatch between looking for images but having to search with words. Creative Commons does have a lot more than images, including video, text, audio (including music), and educational materials, and is a great resource in general.
Luckily, some sites that specialize in images have integrated some form of indexing into their search capabilities. One such site is Flickr. Flickr is a photo sharing site that allows users to assign a Creative Commons license to their images, and add keyword tags describing each photo. You can search by CC license type, then by keyword tag from the Flickr/CC page. Note: there is no obvious way of getting to this search page from the main Flickr page without logging in. However, you do not have to have an account in order to search. Searching can be a little awkward, since you first pick the license (click on See More under each license type), then enter the keyword(s).
There are also a number of sites that are dedicated to providing images.
- YotoPhoto is a search engine for photos licensed under a variety of permissive licenses. They claim over 100,000 images.
- The Stock.XCHNG is a service for photographers to share their work, and also has over 100,000 photos. Be sure to check the license terms, though most will be fine for educational use. You can search by keyword or browse by category. It is sometimes hard to search because of high traffic.
- The Morgue File is a somewhat smaller site (55,000 images) named after the traditional storage collections from newspapers.
- Gimp-Savvy has a photo archive taken mostly from 3 U.S. government collections, NASA, NOAA, and the Fish and Wildlife Service. Most U.S. federal government materials are in the public domain by law, so finding a likely agency (try FirstGov) and looking for images is also a good bet. For instance, the USGS has great maps. And don't forget...
- The Library of Congress' American Memory Project with thousands of images, videos, and audio from the LOC's collections. I especially love the historical maps.
- Free Biomedical Images has over 22,000 images in the biomedical fields, gleaned from open access articles, posted under Creative Commons licenses.
- FromOldBooks warms this librarian's heart, having scanned over 800 images from public domain works. There are even images of old books.
- OurMedia is an interesting new experiment in grassroots media production. OurMedia contains text, images, audio, and video. Unfortunately, the searching is still somewhat primitive. You can search all formats by keyword or browse by media type, but not limit a search by media type. Given the plethora of photo sharing sites on the web these days, I suspect that the static image selection is a little slim, but it does have a nice collection of the more memory intensive audio and video.
On a related note, I've had a couple of requests for video recently. I've been experimenting with Blinkx.tv, a search engine for video. It covers many news organizations, repositories (including OurMedia mentioned above), and video blogs (vlogs). The search is a little funky (I still can't figure out how the refining works) but it looks promising. Google Video and Yahoo Video are also good search engines. Be aware, however, that many video sites only allow streaming not downloads. It may be difficult to incorporate a video clip into a presentation unless you have a live internet connection available.
Update (12/1/05): Someone pointed out that I had missed a subscribed database, the AP Photo Archive, SCSU access link here. NOTE: The subscriber agreement is for strictly educational use. We are also in the process of finalizing a contract for ARTstor, a wonderful database of art and architecture images from museums and slide collections all over the world, produced by the makers of JSTOR. I'll post to this blog when everything is ready to go with ARTstor.