Photos for Class is a very good web tool that you and your students can use to search for and download Creative Common licensed images. The search engine of Photos for Class uses Flickr safe search to return results that are licensed for school use.
What I really like the most about this web tool is the fact that it makes it easy for you to properly attribute images particularly for printed or presented materials. Any picture you download from Photos for Class will have a watermarked section that contains the name of the author, the name of the photo, a link to the original photo, the name and type of license along with a link to read it. No worry about plagiarism or stolen work any more.