Here’s a site to check out.
http://www.rogercavanagh.com/Actions/07_signature.stm The authors make it easy to do that.
It works through menu: File-Scripts.
You get two scripts. One is an ‘initializer’ (VS-Sig-Int) in which you can set color, opacity, position, font, all that good stuff and save it as a named configuration for future use or modification. Thereafter you use script VS-Sig-Batch against any number of files to watermark. Done!
Or you could surf for one of the many alternatives. But try the one above first.