Google Image Search can also filter for re-use licences of the images. I'm wondering how they know which licence an image is published under. How can I declare that licence on my website? Is it possible to declare a licence for each image on the page, or only for the entire page including all referenced images (possibly including pre-fetched content)? And what are the licences that Google understands and can classify to their filter?
I've searched around for a while and have finally found a solution, thanks to Creative Commons. In short, for Google (and other search engines) to know what license the content on a specific page is under, you have to tell it.
This is done the same way as you give Google other data like page relationships - using the HTML structure of the page. In this case, you use the rel attribute of a tags. To declare a single-page license:
<a href="license-url" rel="license">License</a>
Of course you can change the link text to whatever, but the important bit is the rel attribute. The href should point to the license itself.
I don't know how Google knows what license it is, but that's how you declare it, and Google's robots will do the magic for you. In terms of bulk licensing, I dare say you could preprocess pages with PHP (possibly in conjunction with an SQL database) to insert this license tag.
Sources:
Creative Commons Licence Chooser;
MicroFormats' RelLicense
You can also have a look at Sitepoint's definition of the rel attribute and its uses.
Hope this helps.