Hey kleinknecht!
Thanks for stopping by the forum. It would indeed be possible to add that image to your form, and I’m happy to explain how.
To add an image to the header/sub-header section, you’ll actually want to include an <img> element with an absolute path for that image within the settings of the plugin. To do that, open the Settings dropdown and select MailChimp Setup. From there, locate the Content Options heading, and add the <img> tag there, either to the Header or Subheader section.
Here’s an example image URL with the associated <img> tag:
https://static.mailchimp.com/web/features/icon-mailchimp-editor.png
<img src="https://static.mailchimp.com/web/features/icon-mailchimp-editor.png" />
And here’s a quick screenshot of that image now in the sub-header: https://screencast.com/t/5PzjQXgUB
Let us know if you have any questions!
-Elliot