The best option and simplest to implement, is to allow upload of an image and to associate the image directly to the object being edited.
The way I have been doing this lately is to add the image to the content repository and then set the parent_id of the cr_item for the image to the object being edited.
This is much simpler than trying to guess which file storage instance to use. I guess the only issue would be defining a URL for this type of image. We could use the /o/ object redirector for this.