Since we could not find a simple solution online we just finished coding our own little plugin. I am sure we were not the only ones with this problem, so this plugin might help other webmasters too: https://wordpress.org/plugins/lightweight-image-credits/
It’s a simple plug-in, that will add a box to your backend edit pages to insert image credits. A shortcode will show all image credits including the url the image appeared on on a single page.