You can add a watermark to hundreds of images at a time in photoshop if you know how to do it. All you have to do, is go to your actions window, create a new action, then start recording (hit the little record button on the bottom of the actions window)

then you open a picture, paste your pre-made watermark .png wherever you want, then hit ctrl+alt+shift+s to save for web and save it whatever size you want, then hit close, and stop recording.

then you want to go to file>automate>batch.

choose a source folder
select: override action open, suppress file open options dialogs, and suppress color profile warnings, then say "log erors to file, and select a save destination.

then hit OK and it will add a watermark to all of the images in the source folder and save them to the same destination folder you chose before.

More good tutorials can be found at http://technohelpnow.com