Ah I see. So the watermark as you have noticed is not really used. Now it is good to mention that from revision to revision we do change things a bit, so it is possible that it is no longer requested in the stable (r30) or the latest (r32).
Now I believe that you are using this one: https://github.com/Ziggeo/ziggeo-client-sdk/
If so then the testing-watermark is being called in from the dark side by the CSS file, and since it is missing, it is throwing you an error, while it is not really being used at all.
So what you should do is just locate this file: https://github.com/Ziggeo/ziggeo-client-sdk/blob/master/src/css/testing.css
then comment out or remove the line with the watermark being set for background image. Once done that should be it.
* You will need to re-compile things to have your code updated, however no further things would be needed.
Hope that helps :)