If you are using Photoshop you can record yourself creating the watermark and save it as an action, then you can play it back on any photo and it will do it automatically.
If you don't have Photoshop, I have seen something out there that does what you want but you will have to Yahoo it (other search engines are available) LOL
GIMP or Photoshop will be time consuming for you
here is open souce software (free to use ) which do the trick for you
and softwares adds really very strong watermarks
that no one can remove.
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.