Google Image Search, usually found abbreviated GIS around the Internet, is one of the most powerful search tools available if used properly. Available by clicking "images" on http://www.google.com or direct access from http://images.google.com, the GIS is essential for finding raw material for image manipulation, but also has some non-obvious uses.
Most features of the image search are the same as searching for web sites: quotes search for phrases, a minus sign excludes certain results, and advanced search options are available to narrow down a huge selection of choices. Filtering is even available to keep your searches "safe for work." Results are displayed as small preview icons, and clicking them brings you to a framed page which makes it easy to find your image.
When searching a large web page for the text search result which Google found, all browsers have a "find on page" option, usually accessed with ctrl-f or Apple-f. But there is no find on page option for images, and this is where Google's framed page comes in handy. In the bottom frame is the web page that contains the image. The top frame contains the preview icon of the image you're looking for and a link for the web page displayed in the bottom frame. Clicking the link displays the web page without frames. Clicking the image icon displays the image by itself. However, this does not work on some web servers, particularly free hosting services such as Tripod, which don't allow remote linking of images. In this case the entire web page must be viewed to see the image.
Using GIS to search for images
An advanced search might be the best bet for finding raw materials for a Photoshop contest. Larger images are of course better (more resolution to work with provides a more professional result, and it can always be trimmed down if necessary), and one of the search options is "Size". Results are based on pixel size, not file size in bytes. Small images seem to be under 100 pixels in either dimension, Medium 100-1000 pixels, and Large over 1000 pixels. Other advanced options are File Types (JPG, GIF, PNG, or all), Coloration (color, grayscale, B&W, or all), Domain (for specifying a particular site only), and SafeSearch (filtering options).
As is wont to happen around the World Wide Web, Google will sometimes bring up a page that still exists in its database but no longer exists on the Internet. If this happens when searching for a web page, it usually isn't a big deal. Google keeps copies of many old web pages in its cache for just such an event. Google does not, howerever, keep copies of the full-sized images in its image searches. If the web page no longer exists, all you can get is the preview icon.
Using GIS when not searching for images
One non-obvious use for the Google Image Search is narrowing down a huge list of search options when you're looking for information and not an actual image. For example, when researching for my writeup on Phenylalanine, Google returned an enormous number of pages that mentioned the amino acid but provided no technical information. By performing a Google Image Search on the chemical, I was able to quickly locate only those web pages which contained a picture of its molecular structure. These web pages, of course, were far more likely to contain the technical and chemical information I was searching for.
Similar weeding out of unwanted results are possible with common words and phrases, or ones that have multiple meanings. For example, searching for information on fairy ring, I ran across multiple web pages about fairies, leprechauns, and other little people. In this case, using the minus sign to exclude results would have been a poor option, because many web pages that contained useful information also mentioned the mythology behind the growths. The Google Image Search allowed me to weed out those web pages which dealt strictly with the fantasy and not with the science.
Have a product in mind you'd like to buy on Amazon.com or eBay but can't remember its name? Google Image Search for a rough description and pick out the ones that look right. Searching for web sites about Homer the author, not Homer the Simpson? Even searching for Homer -Simpson would result in a huge number of pages about the cartoon character. An Image Search will instantly allow you to pick out the pages you're interested in.
OldMiner notes that images found with the GIS are usually covered by copyright. Check that you're not abusing the fair use policy if you're planning to use them in a Photoshop contest. Some Photoshop contestants cite their sources as a manner of course.