When I searched for the easiest way to add such marks to my photos, I came across Easy Sector's free Batch Photo Watermarker software. This free software allows me to add a watermark logo or phrase to my photos individually or in batch!
Some tips I've found helpful while using this program:
1. The program uses photos in their original size. If you plan to display them smaller (like on a blog), you need to make your text big or it will be too small to read. I plan to use a larger watermark next time:
2. Don't put your watermark in the center of your photos. I find that tacky and I'm much less likely to click on images with obtrusive watermarks when I see them on Google Image Search or Pinterest.
3. Put your name or blog name as your watermark if it can be easily Googled. If your name is common or your blog isn't easily found with a simple Google search, adding a URL to your watermark might be a good idea.
4. Set the opacity to a number under 55 so the watermark is somewhat see-through like on professional photos.