For a stress-free approach to this problem:
Never post hi-res images to the web. By this I mean something that could be used to make a good print larger than 8" x 10".
Consider that unauthorized lo-res images (screen-saver quality) that appear on the web with your copyright notice is free advertising.
You are not losing any money with this approach as the people that post those images won't pay for them. They are just advertising for you.
Advertising - sharing pictures across the internet rarely pays the bills.
I have pursued all manner of companies for copyright theft, PGA of America, Scandinavian Press (23 images), even a small food outlet based in France, several other companies - one company has admitted responsibility (27 infrigements) but continues to ignore payment demands.
I have lost track of the amount of money I have recovered over the years but it has been somewhere between £12,000 - £15,000. It is not complicated to pursue offenders and you don't need an overpaid copyright lawyer in straight forward copyright theft cases - just evidence, letters and invoices. Also it helps to provide the offender the chance to admit any further copyright abuses and offer them a discount.
I fail to understand why people don't pursue copyright theft, especially those committed by companies.