The behavior you are observing on our online demos is due to the location of the initially loaded image. This location doesn't have permissions for image manager and we've configured it in this way on purpose in order to preserve the initial RadEditor's demos content from editing. I've prepared a sample project that shows how you could access an initially loaded image with image editor in the attachment. It could be seen on this video:
However it is not possible to obtain this behavior when the images are on a remote location.
I hope you will find this information helpful.
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