Google have added options to the advanced search page on Google Image Search that let you filter images licensed using Creative Commons. The options lets you choose between images that you’re allowed to reuse, images that can be modified or used commercially. If you want to illustrate your web pages or your documents with images from the web, go to advanced image search page. Under the "Usage rights" section, choose one of the four filters from the advanced search page, find the licensing terms and try to respect them. Your results will be restricted to images marked with CC or other licenses.