So the program I use is called GIMP. You can download it from www.gimp.org. It's free and a really great program for editing photos and adding watermarks. It can be complicated and overwhelming for people new to photo editing programs, but if you have questions just check Youtube. Seriously you can find a tutorial for everything! I've been using it for years and I still learned new stuff tonight watching these videos!!
So here is the video that explains watermarks (LINK), but make sure to check out the other videos if you have questions. I've had a problem trying to dock my dialog windows in the program and I found a video that explains it. It's so simple I can't believe I never figured it out on my own lol. So anyway I hope this helps if you've been wanting to learn how to do it or if you keep getting asked how you do it. :D
I do want to mention something that they didn't say in the video. When putting a watermark on your picture make sure it is fairly large and that it covers a portion of the photo like these below...
If you make the watermark too small or you put it in the very corner of your photo people can still take your pictures and crop out the watermark. I know the watermark can distract from your photo, but better safe than sorry in my opinion.
This last one would be very easy to crop out, which is why I put lines on it. :)
I hope someone found this helpful. It has sure made things a lot easier to explain for me.