The plugin allows you to globally apply a watermark to all product images of the shop or only to products of selected categories.
To start creating your watermarks, go to YITH -> Watermark -> Watermarks and click on +Add watermark.
Now choose the type of watermarks from text or custom image.
To learn how to create text watermarks, please go to this page.
To learn how to create image watermarks, please go to this page.