Hi Jeff-
it depends on which computer program(s) you have. Perhaps "watermark" was a misleading term.
What you see below on the "do not copy" Mike's Casino chip below (10th anniversary edition coming soon ) was done using the text tool in Photoshop.
Most PCs running windows come with 'paint' pre-installed. Just open your picture with paint, and use the text tool in that (although the lettering will have a white background), or if you're agile with a mouse you can use a paintbrush tool....so long as it's conspicuously marked. If you don't have either program, you could alternatively place a thin slip of paper across the front of the chip when you scan it with whatever writing you choose...low tech, but it works. The point is to make it difficult for anyone to get rid of your special mark so they (hopefully) move on and steal someone else's scan...or better yet buy their own scanners
Hope that was of some help!
MP