My epub exports are not valid because in the front-cover.xhtml the link to the image is just assets/

My epub exports are not valid because in the front-cover.xhtml the link to the image is just assets/:

<img src="assets/" alt="..." />

The problem must be related to my setup. Does anybody have a suggestion where to check?
I tried to find out whether the .png file gets exported to the temp folder where the whole epub and pdf generation takes place, but did not find the folder.

Any suggestion how I could solve my issue?
Thank you for any suggestion!

Kind regards