To enable this feature, go to our advanced image search page. Under the "Usage rights" section, you can select the type of license you'd like to search for, such as those marked for reuse or even for commercial reuse with modification. Your results will be restricted to images marked with CC or other licenses. Once you confirm the license of the image and make sure that your use will comply with the terms of the license (such as proper attribution to the image's owner), you can reuse the image. Some of you may already see these options, and we'll be rolling this feature out to everyone throughout the day.
I noticed this yesterday - a gesture towards help helping honest people not infringe on others intellectual property rights. I like it, both as a creator of images and a user/modifier of images by others.