I like using Picasa because it's FREE and EASY! In my "other life" I am a photographer and use Adobe's Lightroom for my professional pictures because I add a graphic watermark to my photos. For now, we'll use Picasa.
When you're in your Picasa gallery, select the photo(s) you'd like to watermark. See the screenshot above. To select multiple pictures click the first picture, go to the last and as you click on the last, hold down the "shift" key to highlight them all.
Go up to file,then scroll down to the option "export picture to folder".