How can I prove licensing terms for online content?

There are more and more websites that offer photos, pictures or designs for free or under Creative Commons licenses. How do I make sure that if I use a photo now I can prove that it was under such a license when I downloaded it? If the copyright holder changes the license afterwards, how can I prove the original license? Especially in case of CC, there is usually no watermark in the photo itself, but just a notice or link somewhere on the website.