I used to use either Eyebatch or Thumbs Plus a few years ago when I was running my own porn paysite. I do not know if these softwares are still available.

Eyebatch is a little easier to use and a little more intuitive. Thumbs Plus is a more powerful tool designed to manage images. It also supports the Digimarc adult watermark plugin.

You can do a photoshop action (no plugin required) that will watermark an entire directory or whatever, but honestly, Eyebatch is substantially faster, and I usually run that on a batch while I’m still doing other stuff in Photoshop.

Sorry, I am no longer using such softwares so I can not help you with that.