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.
Yes. Every photo gets the same mark.
No. You get new files.
No.