You're right, we have a global economy now. That said, every country still has a right to its own laws. The good news is there are quite a few treaties in the area of intellectual property law that try to reach commonality on most of the big-picture items. That said, every country has a very slightly different take on what is "public domain" and what is not. For instance, in the U.S. we have First Amendment considerations to deal with that most other countries don't have.
So the direct answer to your question (how do you deal with all the different laws), the answer is to hire lawyers in all the different countries where you expect you might have exposure. There is no other easy answer, in fact there is no easy answer at all.
Counselor Whitaker is correct that a global economy fitted to a world with some 200 sovereign does not fit perfectly. I think the relevant nations are those where the scan is done, and where the website is hosted, although others may differ. The citizenship of the individuals is not so relevant, except to the extent a citizen is practically most amenable to coercion and damage actions where he or she is a citizen.
Some nations do claim extra-territorial effects of their laws, but mostly for piracy (the kind with guns and knives, not scanners and computers) and terrorism.
Information here is general, does not create a lawyer-client relationship, nor a substitute for consulting with an experienced attorney on the specifics of your situation.
Courts in the United States apply the law where the infringement occurs. What does this mean? Suppose you publish an album in the United States of recordings that are in the public domain in the United Kingdom, but would be under copyright protection in the United States. In an infringement action brought in a US court, the infringement action would likely be sustained. I've attached a link to an article I wrote about a case with this fact pattern.
I'm confused by your fact pattern. Does the person or the company that employs the person have anything to do with the creation of the work that you think has fallen into the public domain?
Any answer or other information posted above is general in nature and is not intended, nor should it be construed, as legal advice. This posting does not create an attorney-client relationship between you and the posting attorney, and you are urged to engage a qualified attorney who is licensed to practice in the relevant jurisdiction.
There are several ways to deal with international trademark issues such as public domain vs. infringement, and here are a few:
1. research the differences yourself, for example using INTA resources such as those available at http://www.inta.org/GlobalTrademark/Pages/main.aspx. Cheapest but riskiest
2. hire an attorney in each country, or a multinational firm with offices in the relevant countries such as one list at http://en.wikipedia.org/wiki/List_of_100_largest_law_firms and let them advise. Good but very expensive.
3. hire a trademark attorney in the US and let that attorney coordinate with the various foreign attorneys in the countries of interest, which takes you out of the loop somewhat, but will usually cost less than using an international firm. Good and less expensive.
Where you are just concerned over Canada and the US, the best will be option 3 as your US attorney will know US law and will either know Canadian law on the matter or will know who to contact for Canadian advice. If you are located in Canada, your Canadian trademark attorney will most likely know US law as that is where the bulk of business originates.
So far, this is free to you. Until you pay a fee, I am not your lawyer and you are not my client, so you take any free advice at your sole risk. I am licensed in IL, MO, TX and am a Reg. Pat. Atty. so advice in any other jurisdiction is general advice and should be confirmed with an attorney licensed in that jurisdiction.
If you have a business need for the answers to your various and sundry questions then you need to integrate them into a business scenario and then re-ask them.
If you're simply curious about copyright law for academic reasons then you should read some books about copyright law: http://goo.gl/VspC3
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.