ImageStamper is a free tool “for keeping dated, independently verified copies of license conditions associated with creative commons images.” You can see an example of how it works here. From ImageStamper:

ImageStamper can act as your witness when you inspect the copyright license of an image that you want to download and use. You can ask ImageStamper to look at the web page with the image to independently verify what exact license conditions apply to that image. ImageStamper will visit this webpage from one of its servers and produce an image ‘timestamp’ — a dated record of image contents and of the license conditions that apply to that image. This timestamp will then be permanently stored in your account and you can present it as evidence that you were given appropriate rights to use the image.

You can help with the development of the project (still in BETA) by giving feedback at the ImageStamper forum.