Hi, Stephan.
The Community Events form does have an Image Upload field, like you said. When uploaded, it gets added to the WordPress Media Library and gets set as their event’s Featured Image.
If you delete it from your WordPress Media Library, it’s entirely removed, including from the event as its Featured Image.
If you’d like to fully remove the Image Upload from your Community Events form, you could do a template override (see our Themer’s Guide) for this file: /wp-content/plugins/the-events-calendar-community-events/src/views/community/edit-event.php
Then remove or comment out this line: tribe_get_template_part( 'community/modules/image' );
Please let me know if you have any follow-up questions on this topic.