I use iView Mediapro to catalog all my images. This wonderfull program has an option to generate pictures and merge them with a watermark file of your choice.
Personally I use this to have a transparant watermark in the image instead of below the image. You can also do that with Photoshop. You can even create an action to do that with just one mouseclick. or you can do it in a btach to all images in a certain directory.