Picture Shark does it all.

http://www.watermark.studigotchi.com/index.htm

Once you create a "stamp" image and after you get going with the settings, yes you can do a whole directory of images at once.

You will find out that if the pictures are about the same resolution, it works best. If you use the stamp on a large picture, it will be smaller than on a mediaum picture.

The stamp is done in pixels, so the size of the picture you are stamping with your watermark, will change the appearance of the watermark.

Not a big deal, but just so you know in advance. I just made four different size stamps. One for 400 x ?? images, one for 640 x 480, one for 1280 x 960, one for 2560 x 1920 and larger.

Point is, it will take some playing with the software and creating your personal transparent GIF (works with others, but this is best), getting the settings right, and then "flip the switch" and it will do all of the selected images.

Here's an example. The idea is to (C) the image and not destroy it. However you can put it so it writes across the middle or whatever you want, and blend it into the image.