Using a watermark may not prevent all image thefts but it will certainly make it harder for someone trying to use your photo.
If you choose the style of your watermark wisely (or even have it done by a graphic designer) it could even add a nice touch to your image. This video by Aaron Nace from Phlearn is a great tutorial on how you can adjust your watermark to fit ANY of your images perfectly. No more tackiness!
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Does this require an advanced Photoshop application or can it be done with regular Photoshop?