Google announced a new feature for Image Search today that should make it easier for you to find, modify and reuse images from across the web.
Google Image Search now has a license filter which will allow you to filter out images based on the license type. This makes it much easier to find public domain or Creative Commons licensed images to reuse or modify.
To access the license filter you have to go to the Advanced Image Search options. At the bottom of the page you will see an option called usage rights with a dropdown list with the options to return images labelled for reuse, labelled for commercial reuse, labelled for reuse and modification and labelled for commercial reuse and modification.