Watermarking is done in the process of manufacturing paper. What appears as a watermark in the finished paper starts out as a pattern applied to the surface of an otherwise smooth roll against which the paper is pressed while it is still wet. That's why when you purchase high quality art paper for alternative process printing, the watermark repeats at regular intervals - the distance between watermarks is actually the circumference of the watermarking roll.

In the US, both Staples or Office Max (and perhaps Barnes and Noble) sell embossing seals. I have one that says "From the library of - - -", and I also have several that I used to satisfy legal sealing requirements as a professional engineer.

Most embossing seals are circular, and are large and rather ugly. I think that if you got a smaller seal, perhaps rectangular rather than circular, and used it on the wide border of a print it could be an attractive way to certify that a print is an original.

In concept, using a seal is not all that different from a Chinese chop.