please insert at least two pictures from somewhere to your Word Online Document. Then, run the Accessibility Checker from the Review Tab - in German Barrierefreiheitsprüfung from Überprüfen.
The word picture is untranslated to German. I suggest to use the same word Grafik as in Word 2016.
Also, in Word Online (English or German), the inserted pictures are not numbered, in Word 2016 Desktop they are. Please also see the screenshots below.