Image Search is a service for browsing, search, and retrieving digital images from a database of images, by the context of the image.
Image search keyword context is typically utilized via metadata, exif data, captions, keywords, alt attributes, and the context of the content around the image and in any anchor elements pointing to the image.
Optimizing images for search is often time-consuming (especially if the optimization plan isn't coordinated with best practices, like naming conventions, etc.), is more often than not omitted or simply an after thought in the web development process.
Image Searches have been proven to drive traffic, and not just for image URLs; engaged users can/will deveate from the image URL and browse the domain that caught their attention/resonated with their search.
Google Image Search is by far the most popular image search service, however there are a few more:
http://www.picsearch.com/ Yahoo Image Search
https://images.search.yahoo.com/ Bing Images
flickr Creative Commons Licensed Images Search
Creative Commons Search Tool (Not 100% Images)