When you say, foreign, remember that copyright violations are pretty common in the USA too, both by individuals and by organizations. From the perspective of most citizens of this planet, the USA is also a foreign country :-) (and GPL violations happen without regard for national borders, too)
Is is harder to take action against people in other countries. You may have to travel there to appear in a court, in extreme cases, and you may have to demonstrate financial losses as a result of the use of your image.
Many countries give a legal moral right to be identified as the author/creator of a work, and also give the creator ongoing rights to say how the work can be used. This may strengthen the poster's case here, although in the US, using creative commons may be seen as waiving some of those rights. Here in Canada, the rights are inalienable: you can't ever get rid of them, which in principle may not be compatible with some creative comments licenses (and GPL for that matter): there's no "public domain" in the same legal sense as in the USA.
Write a letter in general terms in the first instance - e.g., "I am writing because you are making commercial use of one of my images without permission; who would I contact in this matter?" Be firm but very polite at all times.
If you are prepared to settle for acknowledgment, and perhaps a small payment to compensate you for your time, then when you do get a reply, be polite and accept their offer if it's in the right ballpark, or negotiate for a little more. If the reply is unsatisfactory, immediately seek legal advice from a lawyer who specializes in cross-border/international copyright disputes. They are expensive, but you should get a free consultation that will get you started.
Do not be rude, arrogant, or demanding - not only is it likely to make people act defensively, rather than trying to cooperate with you in finding a friendly ("amicable') solution, but it can also actually weaken your case if you do end up with legal action. Similarly, be terse, don't volunteer information. Saying "I don't have much money, I'm a student" for example is also saying "I can't afford to sue you, you can do what you like!"
Do not attempt to base any sort of argument on the Wikipedia pages on copyright; every time I look at them I find errors (often with people fiercely defending them), and I'm not even a lawyer. Reading the actual copyright acts is difficult without legal training - e.g. knowing that a phrase like "time shall be of the essence" in US law might mean "if you don't make the deadline, all bets are off", or finding a footnote on page 50 that says, "hereafter, and everywhere in this document, the term "Ship" shall mean "Ship or hovercraft", or discovering some other law that amends the one you were reading... and even after reading the law, what matters is how individual judges ("courts") interpret it. Sort of like how different Web browsers react to the syntax errors that riddle most HTML pages - the specification says one thing, people do another, the Web browsers resolve it. Except that judges are human, of course, and consider each case as it happens. This isn't so much a criticism of Wikipedia as a note that, like any other resource, you have to know its strengths and weaknesses. For that matter I've seen official government web pages on copyrights that had serious errors in them (such as giving incorrect figures for duration, and then a while later silently changing them!) so it's all a bit of a minefield.