Thread can be closed.
Found a solution after reading through the rasa core sdk source code.
dispatcher.utter_template("utter_video", tracker, False, image = "https://i.imgur.com/nGF1K8f.jpg")
Does exactly what I needed. Maybe a hint in the documentation of this function would be useful for other people since I discovered some threads with a problem similiar to mine when first googling for the solution.