If I'm not mistaken, Photoshop has had "Digimarc" watermarking in it since forever (under Filters). You only get a "demo" ID in the base install (you have to pay for your own personal ID), but the demo would likely be sufficient since you're only really interested in watermarked sample vs. original work. The watermark itself is practically invisible, and meant to be resistant to the sorts of minor edits you're talking about.

The article indicates that there were no trades at all, just large numbers of orders.

What about page files/swap space, application generated temporary files etc. There are plenty of places that potentially sensitive information could leak into on just about any OS.