I gave it a try in a theme component. It’s just proof of concept, so it’s basic. 
For every upload, a watermark is applied automatically.
On the technical side, like how Discourse preprocesses images before uploading (such as compressing), this tool does the same with watermarking. Thanks to the API and Uppy library, watermarking is easily achievable. 
Let me know if you have features/customization in mind that you would consider as a minimum. 
EDIT: will post a version as soon I implement basic settings.