Adding guiding text to the 'Featured Image'-box in the backend

If you are using the Classic Editor of wordpress, there is a function for this:
https://developer.wordpress.org/reference/hooks/admin_post_thumbnail_html/

So the code would be (example from link):

function filter_featured_image_admin_text( $content, $post_id, $thumbnail_id ){
    $help_text="<p>" . __( 'Please use an image that is 1170 pixels wide x 658
    pixels tall.', 'my_textdomain' ) . '</p>';
    return $help_text . $content;
}
add_filter( 'admin_post_thumbnail_html', 'filter_featured_image_admin_text', 10, 3 );

Remember to set the textdomain of your theme.

If you are using the Gutenberg editor, this is a litte more complex. The filter does not seem to work for gutenberg.
As it is made with React JS, I assume it is working differently. Maybe this link can help you:
https://github.com/WordPress/gutenberg/tree/master/packages/editor/src/components/post-featured-image#postfeaturedimage

Moreover I found something that can really help you:

https://thatdevgirl.com/blog/wordpress-featured-image-and-gutenberg

This tutorial can guide you how to adjust the featured image box with filters for gutenberg editor.

Hope this is a starting point for you, if you want to use Gutenberg.

You can also create a custom field of the type of image (Plugin -> Advanced Custom Fields) and use this for the image output in your template. This way you can style the box as you want, add description or whatever you like.

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