title: Codesandbox thumbnail configuration
body: <p>I have some codesandboxes showing the default-sandbox-thumbnail and others showing the thumbnail of the codesandbox right.</p>
  <p>See the screenshot of my sandboxes list<a href="https://i.stack.imgur.com/p2tcB.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/p2tcB.png" alt="enter image description here" /></a></p>
  <p>It is not clear to me from what is depending</p>
  <p>I have looked at the <a href="https://codesandbox.io/docs/configuration#sandbox-configuration" rel="nofollow noreferrer">documentation</a> to understand if i have to configure the <code>sandbox.config.json</code> but i have not found anything useful</p>
  <p>How can i configure the thumbnail of my codesandboxes?</p>