P.S. I was not aware of this DR when I wrote the explanation above. I've, however, found the corresponding OTRS-ticket (see ticket:2010030110058288) where Fæ forwarded the Flickr mail correspondence with the copyright holder. It shows exactly how such exchanges look like when OTRS processes and the requirements of a formal license declaration are not known. But most of these cases get caught through tagging or deletion requests like this one. In this case the license problem was apparently solved by the copyright holder tagging that image with cc-by-sa at Flickr.