Before passing the Parts entity– page parameter to page , you need create a draft Image object as a place holder entity and associate it with the parts. Right now there is no object to hold the Image.
Also it is advised that use one custom image entity with Generalization from the System.Image entity instead of using the core system entities.
The solution is, that I forgot create an new NewImage. On the other hand, it only works, when I commit the NewImage. I guess, that’s a little bit tricky, when my user presses the cancel button for the NewPart… I need to delete the empy ImageObject with an microflow?