It's not entirely clear to me why you don't have the parent entity available if you need it on the phone.
If you have the parent (owner of the association) in the Native profile you can set it in the nanoflow and sync it to the server together with the newly created image.
If the issue is that you create a new image and you don't have the parent entity available then you can consider using an after commit event to set the association between parent and child image on the server.