image description

When you're about to insert the image into your content, you'll see some options above the "Insert into post" button.

You can add your description to the "Caption" field.

sorry, i mean the fautered Image at the top from a article.

Ah, not an easy thing to do, but it is possible: https://generatepress.com/forums/topic/caption-for-post-thumbnails/

Where can i find this? single.php, content.php?

<?php the_post_thumbnail(''); ?>

Those are in the root folder of the core theme, you can copy them to your child theme if you'd like to alter them.

However, none of the solutions in the topic I linked to alter those files - all of those solution simply require you to add the functions using the instructions on this page: https://generatepress.com/knowledgebase/adding-php-functions/