I had a subdomain (e.g. demo.domain.de ) with the plugin installed.
If i want the plugin to set images as featured that are locate in an subdirectory of the superior domain (e.g. domain.de/images/users/image.jpg), there are no images found in the preview listing.
I REALLY tried all options before posting this error.
If you say you tried all options then it is strange. I took a look at your site and could not see any obstacles to find the external images. May I take a look in the backend? If yes please mail me the login credentials to firstname.lastname@example.org.