How to mark the copyright of an image in html?

Question 1

In general

The copyright notice should be contained in a small element:

[…] typically features disclaimers, caveats, legal restrictions, or copyrights.

If the copyright notice applies to the content of a sectioning content element (e.g., article), include the small element in a footer element that belongs to this sectioning element:

A footer typically contains information about its section such as who wrote it, links to related documents, copyright data, and the like.

If it applies to the whole page, you might include the small element in a footer that belongs to the body (i.e. that is not nested in a sectioning element).

<body>

 <article>
    <footer>
      <small><!-- if the license applies to the content of this article --></small>
    </footer>
  </article>

  <footer>
    <small><!-- if the license applies to the content of the whole page --></small>
  </footer>

</body>

When it contains a link to the license

If the copyright notice includes a link to a license (or to a page explaining the terms in more detail), use the license link type, e.g.:

<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 then apply (only) to the whole main content of the page (indicated by the main element).

In your case

figure is a sectioning root element, which means that footer (and header) can apply to it:

The footer element represents a footer for its nearest ancestor sectioning content or sectioning root element.

So you could include a footer element (containing a small element) in the figure:

<figure>
  <img src="img/content/preview.jpg" alt="Alttext für das Bild" />
  <footer><small><!-- copyright noice --></small></footer>
  <figcaption>Caption goes here</figcaption>
</figure>

Structurally, this copyright notice would apply to the whole content of the figure element. (It’s also possible to include the footer in the figcaption.)

(And if you have a link to the license, only use the license link type if this figure is the main content and there is no other main content that should not be licensed under the same license.)

Leaving plain HTML: structured data

By using structured data (e.g., RDFa, Microdata), you can be more specific. This would allow to specify a different license for each piece of the webpage. Example in this Webmasters SE answer.

Question 2

You should use  (HTML5 Spec):

Small print typically features disclaimers, caveats, legal restrictions, or copyrights. Small print is also sometimes used for attribution, or for satisfying licensing requirements.

Question 3

I've seen the use of Your copyright text for this purpose.

From the HTML spec on 

The small element represents side comments such as small print.

Small print typically features disclaimers, caveats, legal restrictions, or copyrights. Small print is also sometimes used for attribution, or for satisfying licensing requirements.

Previously the  element was used to display a certain text with a smaller font, but nowadays all styling should be accomplished through CSS, and your HTML should only act as what the name suggest - markup.

Your browsers default stylesheet might still show the content of a  element with a smaller font-size though, so you might have to override that with your own CSS.