For an explanation, see this blog post: Semantic markup for “You can license this image”

Plain RDFa and the rel=license microformat

Example photo #1 by Tim Strehle (CC License).

<div about="http://www.strehle.de/tim/semtest/photo1.jpg">
  <img src="http://www.strehle.de/tim/semtest/photo1.jpg" />
  <span property="dc:title">Example photo #1</span>
  by <span property="dc:creator">Tim Strehle</span>
  (<a rel="license" href="http://creativecommons.org/licenses/by/3.0/de/">CC License</a>).
</div>

WhatWG microdata (“Licensing works”)

Example photo #2 by (CC License).

<figure itemscope="itemscope" itemtype="http://n.whatwg.org/work">
  <img itemprop="work" src="http://www.strehle.de/tim/semtest/photo2.jpg" />
  <figcaption>
    <span itemprop="title">Example photo #2</span>
    by <span itemprop="author">Tim Strehle</span>
    (<a itemprop="license" href="http://creativecommons.org/licenses/by/3.0/de/">CC License</a>).
  </figcaption>
</figure>

schema.org + rNews, in microdata format

Example photo #3 by Tim Strehle (CC License).

<div itemscope="itemscope" itemtype="http://schema.org/ImageObject">
  <img itemprop="contentUrl" src="http://www.strehle.de/tim/semtest/photo3.jpg" />
  <span itemprop="name">Example photo #3</span>
  by <span itemprop="creator">Tim Strehle</span>
  (<a itemprop="usageTerms" href="http://creativecommons.org/licenses/by/3.0/de/">CC License</a>).
</div>

schema.org + rNews, in RDFa Lite format

Example photo #4 by Tim Strehle (CC License).

<div vocab="schema.org/" about="http://www.strehle.de/tim/semtest/photo4.jpg" typeof="ImageObject">
  <img property="contentUrl" src="http://www.strehle.de/tim/semtest/photo4.jpg" />
  <span property="name">Example photo #4</span>
  by <span property="creator">Tim Strehle</span>
  (<a property="usageTerms" href="http://creativecommons.org/licenses/by/3.0/de/">CC License</a>).
</div>