This page of the documentation will help you with the creation of a new watermark using the premium version of the plugin. Follow the procedure to discover all settings needed to customize your watermark.
The plugin lets you assign a watermark at the same time on all products of the shop, on all products of selected categories, or in a customized way for each available product. In the first case, you will have to create watermarks in the “Active Watermark” tab available in the settings panel YITH Plugins -> Watermark.
On the contrary, if you want to overwrite the single product general rules, you will just have to open the page of a product and enable the “Watermark” entry. From this moment on, you will be free to configure different watermarks only for that specific product.
The plugin creates a backup of all product images and the files are stored in the wp-content/uploads/yith_watermark_backup.