Hi Gord,
Thanks for posting this warning, it never ceases to amaze me the scams people invent.
Watermarking images is possible, though there are all sorts of problems associated with it.
Visible watermarking has several disadvantages and limitations. If implemented in a non-intrusive way, it can probably be easily circumvented by the scammer.
Good invisible watermarking costs money to implement, although I believe reasonable shareware and lower cost software options now exist. The problem here is the would-be buyer needs a means to check for the invisible watermark – usually the software that created the watermark in the first place. This probably means asking us to check each advertiser’s images for an invisible watermark.
So when I do a quick mental run-through of these image protection options, a non-intrusive but secure watermarking operation turns into a big job, well beyond the means of a free community site like this one.
Your advice about adopting a suspicious mindset when dealing with unknown advertisers and doing appropriate due diligence, is probably the best suggestion and something we should write up in an advisory post for Members. Perhaps you could tell us what kind of searches you did to trace the true source of the photographs?
Dave