Stamping a photo with a watermark

So I wanted to stamp a large number of photos with a watermark, so I could share printed copies with people.
ie.

EinFesteBurg

I did it by using FastStone Image Viewer “Tools\Batch Convert Selected Images” option.  I then clicked Advanced.
FastStone Image Viewer
Notes on Font size:

Image width in pixels Font size
150 5
450-900 16
700-1200 24
2400-6000 48
6000-8000 72