The plugin “BJ Lazy Load” is to blame. deactivating this plugin fixes the issue.
However, I’m not sure yet if only deactivating this plugin might have solved the issue, or if it is a combination of both, deactivating BJ Lazy Load and adding your suggested CSS line to the custom css option of my wordpress theme.
I’m happy though, it now looks great again and you did a great job. Thank you so much!