Watermarks have been around forever. Before digital, it was a photographer's signature in the corner of a print. Then when the digital world arrived, you would find a watermark anywhere on a photograph.
We won’t get too much into the specifics of what is going on with this Virginia Federal Court case in which the court found that using images that you find online can be constituted as fair use. Chances are you have heard about this already, or if you haven’t, a simple search can give you all of the information about it really quickly.