Not enough information. First, since you mentioned Photoshop, can I assume you have Photoshop? If you do, I would recommend a PS Action. Once it's set up the first time, one click should fill all your watermark needs. Note that you might need to make one for landscape mode and one for portrait mode. Also, PS comes with something called Digimarc, at least I think it comes with it because I don't remember installing it myself.
For PS Actions...
http://www.google.com/search?aq=f&sourceid=chrome&ie=UTF-8&q=photoshop+watermark+actionOr you can try one of these. These are programs that only do watermarking. If you prefer free, just add the word free to the search box...
http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=quick+easy+photo+watermarkOr even better, these are free online tools that allow you to watermark...
http://www.google.com/search?aq=f&sourceid=chrome&ie=UTF-8&q=free+photo+watermark+onlineOK, now maybe you're thinking, TOO much information! :)
Good luck!
BTW, I can't vouch for any of these.