Most images on Google are copyrighted which gives the owner a right of copy that can limit a user of those images freedom to copy, distribute, use, or modify. Creative Commons came along to grey up the space between No Rights Reserved (Public Domain) and All Rights Reserved (Copyright). Creative Commons allows owners to get very specific in regards to how their images can be used. For example, can the image be used, modified, used in derivative works, or for commercial gain. This comic explains it well.
Creative Commons has created a site, while not a search engine itself, allows you to search for Creative Commons works from major sites such as Google Images, Flickr, You Tube. You can search for commercial (making money off the image) or right to use, modify, and adapt.