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".
This pop-up menu will come up. Up near the top, just click where you want your watermarked pictures to go. Down on the bottom there is a spot where you can check "watermark". Click the box, then right below it you can type in your web address/blog name, etc.
...and THAT'S IT! I learned the hard way, I hope this helps some of you out so you can learn from MY mistake. And please, pin away from my blog. I love seeing the things you extrapolate from my lessons and make into your own!