Google Images has an option to filter images based on license.
Each image on my site has a details page, where I have the following link to denote the license:
<a rel="license" href="http://creativecommons.org/licenses/by/4.0/deed.en_US" target="_blank">CC-BY</a>
It's a modified version of the sample code from Creative Commons - Choose a License.
However, I see images that Google has indexed since I added this code do not appear when I select a license filter.