If you are using Window 7 or other windows versions, try downloading Windows Essential, it's a group of tools and you can find a photo editor in there. It's very easy and convenient to use. Watermark you photos with ease. It's free by the way. Goodluck.
Using Photoshop and other tools like GIMP will prove to be a waste of time.It will take a lot of time to watermark photos using this procedure.The watermarks will be placed at uneven locations if you try to use the Batch Mode.
Online Watermarking tools will consume a lot of bandwidth and you cannot process high quality photos (eg:15mb) uploading and downloading many of these photos will be a waste of time.Also from my experience the features are limited to basic watermarking and only few photos can be watermarked at a time.You need to pay a monthly fee Then.I was fed up of this and finally found a solution i was looking for-Mass Watermark.I have been using this tool it can Watermark,Resize,Optimize,add EXIF Info and finally upload watermarked images to Picasa/Flickr after processing is completed.
Solved all my problems
I have also used the Photoshop for watermark for the photos. But I think the point of the watermark is to, largely, make them unusable. If you wants photos for promo use he has to pay for them like everyone else.