If your photos are worth reposting, they are worth marking. A watermark is the only thing that survives a screenshot.
A small mark in a corner at low opacity is polite and useless. It crops off in seconds. If the point is credit on something you are happy to share, that is fine.
If the point is stopping the picture being used without you, repeat it across the middle, where removing it means damaging the image itself.
A logo with a transparent background sits on a photo far better than one on a white block. Save it as a PNG before bringing it here.
A faintness of 40 to 50 percent is visible without spoiling the picture. If the photos are valuable, use the tiled option, because a mark in the corner can be cropped away in seconds. Try the settings on one light and one dark photo, because a colour that shows on one can vanish on the other.
The size is relative to each photo, so the batch stays consistent. The pictures are saved again at high quality, with marked added to the name. Keep the originals without the mark, and post only the marked ones.
Yes. Every photo gets the same mark.
No. You get new files.
No.
Each photo is saved again at high quality, so any change is very small. Keep the unmarked originals in case you need them.