For a watermark you will need a photo editing program that can deal with "layers".
The way you would do this is:
1-Open the picture file of whatever it is you wish to use as a watermark. Example: watermark.jpg
2- Save the file as "Watermark1" using whatever is the programs native filetype. example: Watermark1.psd for photoshop. doing this leaves the original file intact and unchanged.
3- you will probably need to change the original watermark picture so that it makes a better looking watermark. First, you may want to resize it. Then perhaps apply filters to it like "emboss" or "extrude" to make it kind of 3-D. Maybe make it black & white, whatever.
Save various examples as Watermark1, 2, 3 etc. I suggest leaving the Opacity alone at this point.
4- Open the first picture that you want to watermark, perhaps Photo.jpg Save it as Photo1.psd (or whatever)
5- "Place" the watermark1 picture. "File">"Place">choose file. This may also be called "Import". This will cause the watermark picture to be layered over the original Photo1 picture. Now, resize and move the watermark to the desired location.
6- Adjust the opacity of the "Watermark1" layer until the desired transparency is reached. This will give you a watermark.
You could open the watermark file and adjust its Opacity to this level. Or just set the opacity on every photo after placing the watermark.
7- After the photo is finished, save it in the desired output format. I recommend the original filename plus a suffix to indicate changes were made. example: Photo1 jpg (without watermark) Photo1b.jpg (with watermark)
By making a separate watermark file, you won't have to apply the same filters again and again. Just place it on every photo in the same position.
You could also automate this action but that's a little more involved.
Good Luck,
Keri