The free version of YITH WooCommerce Watermark lets you apply a watermark to the images of all products of your shop. The watermark is visible on every product detail page.
Once activated, the plugin creates a copy of all images of the store (in the “Uploads” folder of WordPress), in order to easily restore them if you want to delete the added watermark.
Click on YITH Plugins -> Watermark to go to the plugin settings panel.
The size of the image you want to apply a watermark to must be the 25% of the original product image (assigned from the settings panel of WooCommerce).
The free version allows you to add a watermark to 15 products at the same time. Once impressed on products, the watermark will be shown in the product detail page, as you can see in the image below.
The plugin creates a backup of all product images and the files are stored in the wp-content/uploads/yith_watermark_backup.