Problem is, that image files cannot be deleted via GUI, since the owner of the files is root (some images are created by a cronjob directly. There are also the conversions generated by medialibrary itself within a Laravel job.).
I did not find a way to tell medialibrary to set a specific owner. There is no method or configuration option for this.
Since medialibrary uses the Laravel filesystem, the configuration file filesystems.php
could theoretically allow specifying an owner for all files in the medialibrary base directory. However, I only found a setting to specify permissions: https://laravel.com/docs/9.x/filesystem#local-files-and-visibility