Even in the outraged article the original poster refers to, the author concludes that the stock image house was within their legal rights to demand payment. Webmasters need to learn that if they didn't create it, the basic rule is they can't use it without permission - whether "it" is an image, a video, an MP3, a section of text, or anything else, and there are real world consequences for ignoring that fact.
Yes they use scare tactics like these - usually removing the picture in question resolves it. There only testing if you will pay up or not, and its very unlikely they will take this further - im not a lawyer tho.
Tell them that you have downloaded from elsewhere (which you probably did) without knowing it is protected by copyright. At this point, you may remove that picture and inform them about the change or, if you really need it, offer your self to buy it at their normal price. Could be no more than 30-40 USD, I presume. In other words, try to solve it amiable.
Anyway, you should ignore the $3000 ask. They will never have the patience to sue you for just a small piece of cake. That sum could be only the tenth of their lawyer's fee and their legal charges, in case of. If they are stubborn, tell the press, make waves about that all over. Their way is nearly an extortion method, so it might turn against them.
I agree with Kor for the most part.
No one's going to pursue you for a thumbnail. First check the legitimacy of the claim because there have been a couple issues with people just claiming they created an image and then demanding payment for it. However, if you just took it from somewhere, you have to guess it belongs to them. If you took it without asking, that is the definition of stealing.
Either way, try to offer to pay normal price for the image, forget about the empty threat, and immediately remove the thumbnail from whatever you used it on.
But, if you actually made money off of using the thumbnail... God help you.
Even if you made money from the thumbnail, there is no way of proving this in court - they are trying their luck to get money. Just remove it and put up a similar one - the risk and costs for them are really high if they actually took legal action against you - law firms only do this when they know there is a good chance of winning.