I solved this now as following:
Incident entity with 1-n to IncidentPicture (IncidentPicture_Incident) entities and a 1-1 to the same entity (IncidentPicture_Incident_Upload)).
In the After create microflow of the Incident entity I create a IncidentPicture and set it as the IncidentPicture_Incident_Upload association.
This Incidentpage has a dataview for the associated IncidentPicture_Incident_Upload with the fileuploader and a upload button which add the IncidentPicture to the 1-n association and set a new IncidentPicture to the 1-1 association.
Works fine.