Generally
The copyright notice must be contained in a small element :
[...] Usually contains disclaimers, disclaimers, legal restrictions or copyrights .
If the copyright notice applies to the content of the sectioning content element (for example, article ), include the small element in the footer element that belongs to this sectioning element:
footer usually contains information about its section, for example, about who wrote it, links to relevant documents, copyright information and the like.
If this applies to the entire page, you can include a small element in the footer that belongs to the body (that is, that is not nested in the sectioning element).
<body> <article> <footer> <small></small> </footer> </article> <footer> <small></small> </footer> </body>
When it contains a link to the license
If the copyright notice contains a link to the license (or to a page explaining the terms in more detail), use the type of link to the license , for example:
<a rel="license" href="/license.html">Usage rights</a>
<a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">CC BY-SA 3.0</a>
But note: this license will apply (only) to all the main content of the page (indicated by the main element ).
In your case
figure is the root element of the partition , which means that footer (and header ) can be applied to it:
The footer element represents the footer for its closest ancestor, sectioning content, or root sectioning element .
Thus, you can include the footer element (containing the small element) on figure :
<figure> <img src="img/content/preview.jpg" alt="Alttext für das Bild" /> <footer><small></small></footer> <figcaption>Caption goes here</figcaption> </figure>
Structurally, this copyright notice will apply to the entire content of the figure element. (It is also possible to include a footer in figcaption .)
(And if you have a link to a license, use only a communication type license if this figure is the main content and there is no other main content that should not be licensed under the same license).
Leaving Simple HTML: Structured Data
Using structured data (e.g. RDFa, Microdata), you can be more specific. This will allow you to specify different licenses for each element of the web page. An example in this Webmasters SE answer.