The main idea of this feature is to allow users to say that all content which they upload to a site is licensed under a specific license. This can be combined with Single File Upload Choice to enable users to have a default license for all media, but on special occasions have the ability to change specific files.
Below is what flickr does to enable this feature for their users:
Use similar output to what the Commons License Chooser service provides users. For example, users who choose the Creative Commons Attribution License receive the following code snippet:
<a rel="license" href="http://creativecommons.org/licenses/by/3.0/us/"> <img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by/3.0/us/88x31.png" /> </a> <br />This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/3.0/us/">Creative Commons Attribution 3.0 United States License</a>.
The code will display an icon as well as a link to the full license deed hosted at the Creative Commons site. This is the appropriate way to mark CC-licensed content.
More information about simple marking practices can be found at the wiki Marking page.
RDFa is RDF in attributes and is a good way to exhibit semantic relationships for search engines and other machines. RDFa allows users to annotate human readable notices on individual assets that include the custom properties needed for attribution as well as other useful properties. The Syndication page explains how license statements may be included in various syndication formats, at both the feed and individual item level.
XMP facilitates embedding metadata in files using a subset of RDF. Most notably XMP supports embedding metadata in PDF and other image formats, though it is designed to support nearly any file type.