You can add custom watermarks to specific or category of products by using WooCommerce watermark plugin by Extendons. This plugin helps to add text and choose the font, color, size and also adjust the position. WooCommerce watermark plugin allows you to create the rules to watermark the images, which gives ease to apply the watermark on product pictures. You can specify the rule name and also set the priority for rules. You can get this plugin for just $39 with 6 months free support.