I use a program called GIMP which is an open source (ie...free) photo editing software. Procedure looks something like this:
First, create your watermark. Logo, image, initials, or whatever little "stamp" you want to put on each picture. Create it using a transparent background. This will allow it to easily layer over your pictures and blend in better. I also reccomend playing around with the size a little until it's a good fit for the photo format your camera saves it's pictures in and then save a copy it in that size so you don't have to resize every time. Here's mine:
View attachment 726096
Once you've got your watermark created and saved, here's how I add it to an image.
1. Open the image you want to watermark in GIMP
2. Go to the file menu and select "Open as layers..."
3. Find and open your watermark. After opening it should be floating over your image
4. If the layers toolbox is not visable, use CTRL-L to open it. Adjust the opacity on your watermark layer until it blends in how you like. For me, 50% opacity is usually around where it looks the way I want
5. Use the move tool to position the watermark layer where you want it on the image
6. If you want to keep your original un-edited image and not overwrite it, go to the file menu and select "export as..." and save in the location and format you would like. Otherwise just save back over the original image using "File -> Save" or "File -> Overwrite
imagename"