Result:
(A) On the Fire TV there was MEGA excessive artifacts. Apparently Amazon compressed files mercilessly down for viewing
(B) On the PC browser, it was slightly better than on the Fire TV, but still with extreme compression artifacts
(C) On the Tablet saw it still 'somewhat useful' - but even there, there were significant JPG artifacts.
Conclusion: Amazon Cloud Drive Viewer - nice for undemanding photo users who 'sometimes just a snapshot' want to watch; unusable (at least in the current version on Nov 2014) for users with high demands on image quality during playback.
PS: If you can not do anything more with the term artifact - on Wikipedia, there are in "Artifact (Computer Graphics)" a viable definition.