Originally, themes were supposed to also define watermark to be used in processing. This not just includes uploading (and storing) the actual watermark, but also the ability to set a position of the watermark.
While this feature has never been completely implemented, the current state of the codebasis is quite close to be able to support watermarks on theme level.
To achieve this task, the following sub-tasks will need to be performed:
Resurrect the form Configuration->Themes->"Theme Details"->Watermark
Make sure that WOH theme provides enough information so that an appropriate workflow definition can implement the processing of watermarks
Adjust publish workflow to actual implement the watermarks as defined in the theme