Hate to tell you...you can watermark them till the cows come home, but the watermark can easily be removed. I use to watermark all of mine..did not matter one little bit....people would post them with my watermark..or would blur out my mark (full store name) with editor. (can't believe they would take that much time to do it but they did). I finally just stopped messing with it.
I would tell you how I did it, but honestly it has been so long I really do not remember process.
I do report theft to ebay via their report form AND call safety dept daily until they are removed. Recently had someone steal text and images and reported them 3 separate relistings in a row on same item. Ebay would remove, they would relist same way...3 times and they are still selling away...but so far have not relisted that item a 4th time.
Frustrating to say the least...as honestly my photos are OK, but they are really not that much to write home about!
Good luck and sorry I just do not remember how I did it before it has been so long.
It is frustrating to say the least!!!