Assume you use some images with a Creative Commons license. The creative commons license states that the license cannot be revoked if you acquired the images while published as Creative Commons.
However, the owner may republish the images using a completely different license. While this does not affect your existing use, it might be necessary to prove under which license you had the images acquired.
I am aware of ImageStamper, which offers a service for Flickr images. I'd look for a more general solution, ideally without the risk of some provider going out of business.
Some notes to clarify my question:
I look for a tool to prove that I have obtained an image under a certain license (and from whom). This is important to prove if the owner claims later it had never been under this license. The tool doesn't need to prove where I got them from.
This can also help to protect me from someone "licensing" a work that is not his own (with some risk left, but as the work was never his, he's usually liable).
It does not matter if the image is under multiple licenses. I only need (and care) for the one I obtained it.
Thanks for your help :-)